Still working on an Doug Hamilton has a nifty Astronomy Workshop web page, with lots of fun little tools.
I had used some of these for my "Stars for Poets" course, but was reminded of them when looking at Bad Astronomy this morning.
Doug has a Generic central force integrator, an issue which came up recently of CosmicVariance (its an orbit-in-a-static-potential integrator, not an N-body tool, but it is a start, you'll get the general idea of the instabilities that occur for non-Newtonian forces).
You can smash a star through the Solar System, or just hit the Earth with your favourite asteroidal or comet impactor - my favourite
