UK/ Ireland in August?

I'm not sure what kind of things you want to do but here are a few suggestions for Dublin (we don't have much of a fly problem, you're more likely to get rained on!).

If you like museums, the Natural History museum on Merrion Square has loads of animal specimens (skeletons and stuffed). If you've the kids with you, they might enjoy that one. The Irish history museum on Kildare Street is worth a look too, if that's your thing.

The tours of Farmleigh House in the Phoenix Park are supposed to be great (of course as a local, I've never been :) ). It's a fully restored old Georgian House with gardens.

You'll have to have a pint in a nice pub. Avoid Temple Bar like the plague (especially at night). O'Donohue's on Merrion Row or Doheny and Nesbitt's on Baggot Street are nice old fashioned pubs if you're in that area. They're near the museums I mentioned. The tour of the Guinness brewery is really good too and the bar up the top has the best pint of Guinness I've had yet.

Hopefully there' some ideas there.

I should've mentioned that we'll be kid-free for this trip. They'll be visiting the grandparents.

I'll give you a few options for London and daytrips from there.

1. The Essentials: Most of the essential sites are near each other, so this is what I recommend for a one day trip. Start at Westminster Abbey (go early in the morning if you actually want to go inside). Then go to Parliament square to see Big Ben. Next, walk down Whitehall to see no. 10 Downing street. After that, go across Green park to see Buckingham palace. As an optional extra, you can then walk to Picadilly Circus and Leicester Square, finishing off in Trafalgar Square to see Nelson's column. You can of course also get one of those hop-on, hop-off bus tours to see this stuff, but it is all walkable.

2. A sciency day: If you want some sciency goodness then I recommend going to the Grenwich observatory. There you can see an exhibition on the history of timekeeping and stand with one foot on the east and west sides of the Grenwich meridian. Grenwich also has the Cutty Sark (a cool ship) and the Royal Maratime Museum (one of the most boring museums I have ever visited). You can get there via a boat trip on the Thames or via the Docklands light railway. I recommend taking the boat one way and the railway the other. The light railway is worth it just for the odd East London station names like Mudchute.

3. Museums: If you want a day of museums then go to South Kensington. There you have the Victoria and Albert museum, the Natural History museum, and the Science museum. The V&A is the one that tourists usually gush over, but I have a soft spot for the Natural History museum. You probably don't need to see another science museum, but it is a good one.

4. Art galleries: The two essential art galleries are the National Gallery on Trafalgar Square and the Tate Modern on the South Bank. You may be able to squeeze the former into "the Essentials" tour, and for the latter...

5. The South Bank, Tower Bridge, and the Tower of London: It is pleasant to walk along the South Bank of the Thames from around Waterloo/Embankment station to Tower Bridge. Along the way, you pass the National Theatre and all the other arts/cultutal venues around there, the London Eye (giant overpriced ferris wheel), and the Tate Modern, ending up at the iconic Tower Bridge across the river from the Tower of London. There are street performers and a book market along the way. I enjoy doing this walk in the later afternoon/early evening, but if you want to actually go into the Tate Modern or the Tower of London on this walk then you will have to start earlier. The Tower of London is an essential tour for history buffs, but it gets very busy in the summer.

6. Tourists usually want to go "up" something to get a view of the city. The London Eye is the modern overpriced version. The more traditional option is to go to St. Paul's Cathedral and climb up to the whispering galley, from where you can go up a little further to get an outside view.

7. Pubs, restaurants, etc: If you can, you want to avoid the touristy west end for dinner, drinks, etc. Real Londoners go out in the East End instead these days. If you are staying near the Worldcon venue then this will be just around the corner for you. There are some very good pubs that also serve food around there. Unfortunately, these are often on side streets so not immediately obvious. The best idea is to look at an online guide beforehand so you know where you are going. A favourite of mine is The Gunmaker's Arms for food, and there is also a traditional wood panelled pub nearby that is good for beer, but I forget the name.

8. Shopping: The obvious tourist tat shops are on Oxford Street, so that is where you need to go to get mini Big Bens, toy buses, etc. Unfortunately, you'll probably need to go to one of these shops to get presents for the kids. For something different go to Borough market on the south side of the river, which is a produce market with plenty of free samples. It is a good place to go for lunch.

For day trips from London, three options are Oxford, Cambridge, and Hampton Court. Oxford and Cambridge are somewhat similar, so you probably only want to do one of them. Cambridge is a little nearer and has a direct train from King's Cross that takes 45min. For Oxford, it is best to take the bus from Victoria that takes 1hr30min. The main attractions there are the colleges. In Cambridge, it is worth looking at St. Johns, Trinity and Kings. I'm less familiar with Oxford, but it has the "Harry Potter" college and also a museum of scientific instruments that features a blackboard with some equations written on it by Einstein. In both places, punting on the river is a fun thing to do.

Hampton court is where Henry VIIIth lived and is only a little way outside London. You can get there via a boat trip on the Thames. As well as the historical stuff inside, it also has an impressive garden featuring the only maze I have ever got lost in.

In case you're not aware, the 2014 Eurocon, Shamrokon, is in Dublin the weekend after Loncon 3, so you might be able to catch a second international convention (albeit much smaller) if you're so inclined.

Newgrange is lovely, but perhaps a bit over-organised and over-restored. If you take yourselves off to the west of Ireland, there are smaller passage graves just lying around the hillsides, that you can have to yourselves. If you have any interest in more recent Irish history, Newgrange is very close to the site of the Battle of the Boyne.

London and Dublin both have pretty variable weather in August (and every other month of the year). It won't snow. The temperature will almost certainly be somewhere between 50 and 90 Fahrenheit (a little cooler in Dublin than London). It could be clear blue skies, perpetual grey overcast or torrential thunderstorms (a little wetter in Dublin than London). Or all of the above on the same day.

You could certainly keep yourselves busy indefinitely just in London. Other London attractions not mentioned by previous comments include ships (HMS Belfast and the Golden Hinde replica as well as the Cutty Sark), theatres (Shakespeare's Globe, the National, the Old Vic, the Young Vic, dozens of West End theatres and hundreds of smaller venues), live music (the O2 just across the Thames from Loncon 3 is the most popular music venue in the world), dozens of small museums (e.g. the Fan Museum, the Geffrye Museum, Sir John Soane's House, the Museum of Childhood, and so on), the Royal Opera, Sadlers Wells ballet and Camden Lock market.

I wouldn't necessarily recommend Oxford, Cambridge or Hampton Court as the best day trips from London. Consider also Canterbury, Salisbury, Brighton, Lille, Paris and Brussels. The Loncon 3 site is close to Stratford International station, which doesn't actually have international services but does have high speed domestic services that connect with the international trains at Ebbsfleet International or Ashford International (or at St Pancras International if you don't mind going a few miles in the wrong direction).

London has a reputation for being particularly expensive, so you will have to be careful about expenses. It's the best place outside of India to get Indian food, so it's great if you like that cuisine.

The last time I was there, we did Stonehenge/Salisbury/Bath as a day trip. Southern England is quite geographically compact, so many such combination trips are possible.

If the theatre is your kind of thing, try to catch a West End show. For museums, there is also the British Museum near Russell Square.

If you were thinking about driving in London, terminate that thought with extreme prejudice. Public transit (tube, buses, etc.) goes everywhere you might want to go, and while Brits like to complain about it, it's better than anything I've encountered in the US. Also, Brits have a perverse insistence on manual transmission, so if you can't drive a stick shift, your rental options will be even more limited.

@Eric - Yes, we learn to drive properly over here... ;-)

Don't think I can add much more on sights to see - was going to recommend museums but previous posters already covered that off.

My other tip for London, if schedule allows - just take time to walk about. Particularly if you can take in one of the parks, just spending an afternoon rambling the city, following your nose & seeing what you come across is very pleasant. It also helps getting away from the crowds - and in August, it will be busy.

Obviously somewhat weather dependent - as previously noted, expect anything. The joys of a maritime climate...

If ancient monuments are your thing, I would visit the Salisbury Plain, which is about an hour or so west of London. A visit to Stonehenge is obligatory, but much more interesting is the huge henge and stone circle at Avebury. Here you can climb the banks of the henge and touch the stones. Silbury Hill and the West Kennet Long Barrow are within walking distance. Indeed, you might consider a day's walking in the area. I'm sure you could find a route that links several monuments (with a pub or two along the way). Here is one that the Telegraph has put together: http://www.telegraph.co.uk/travel/activityandadventure/walkingholidays/… . It looks like there is a bus service called the "Henge Hopper" that links all these sites to a railway station on a line from London: http://www.stonehenge-avebury-bus.org.uk/route so you could do it as a day out.

And if you are anywhere near Bristol, please stop by the Physics World office and we'll buy you a pint or two!

Oh dear, it looks like the Henge Hopper no longer runs! However, you can get from the mainline station at Swindon to Avebury via this bus...
http://www.swindonbus.info/tts/049.htm

While Matt Leifer's comments seemed to cover most of the important tourist attractions in London, I would personally choose the British Museum at Russell Square over all the other major museums in London. They're all good, but the British Museum is my favourite by far.

Further endorsement of Avebury over e.g. Stonehenge, and of the British Museum. I would offer a counter-view on the Tower, though: I think it's the best of the major London tourist attractions, even with the forced-pace cramming that is now the Crown Jewels viewing (go early!). Stratford makes for a wonderful day trip; if picking between Oxford & Cambridge, choose Oxford. Greenwich's Observatory is great and unique; the adjacent Cutty Sark is OK-but-not-essential.

Walk or Tube everywhere. Buses are also good and the transport for London website offers good planning tools. Eat Indian food. Enjoy the huge improvement in British microbrews over the last decade.

As for Dublin, other than the glorious Trinity and Book of Kells we were underwhelmed by Dublin, to our surprise; that contrasted with much of the rest of southern Ireland which was gorgeous. But it's been 15 years since we were there, so I'll leave further comment to others.

I spent a month or so doing some research in London, and these were the high points for me:

As far as museums go, I would put the British Museum at the top of the list. After that, in descending order, the Hunterian Museum of the Royal College of Surgeons in Lincoln's Inn Fields, the Natural History Museum, the National Gallery, the Churchill War Rooms in Whitehall, the Tate Modern, the Victoria and Albert Museum, the Imperial War Museum and the Science Museum. (For me, the Science Museum was pretty disappointing, with the exception of Charles Babbage's brain.) The British Library also hosts temporary exhibits which might be interesting (you can also get a library card using your university credentials, which makes for a neat souvenir).

Things to hit on a walking tour of Centralish London that haven't been mentioned yet: Cleopatra's Needle, Seven Dials, Trafalgar Square, Lincoln's Inn Fields, Ye Olde Cheshire Cheese pub, and Waterloo Bridge/Station.

Two things that haven't been mentioned yet: If you do go down to Greenwich, I'd recommend a trip through the Greenwich Foot Tunnel, probably the creepiest way to cross from one side of the Thames to the other. Finally, a couple of long walks through Hampstead Heath and maybe a dip in the swimming ponds there.

Regarding Newgrange, it's only about 35 minutes from Dublin airport so you might want to continue north after your visit for a dramatic change of scenery. The countryside around Newgrange is flat and unremarkable, but if you drive another 30 min or so north you get to the Cooley Mountains and Carlingford Lough -- which is a mountain-lined fjord that comes in from the Irish Sea. The town of Carlingford is worth a visit for its medieval fortifications and the spectacular views across the lough to the Mourne Mountains in County Down. It's the quintessential Irish vista of sea, castles and mountains!

If you're there in August, you can do tours of both the Houses of Parliament and Buckingham Palace, as they're both closed for their primary purpose at that time of year.

You may well prefer to just look at the buildings from the outside - the tours are lengthy, and security-heavy (airline-style, apart from not having to take your shoes off).

The Faraday Museum in London is a small gem that is not often visited, but was a highlight for me.