One is in Boston, the other in D.C. In Boston, the Boston Public Library has an amazing collection of travel posters from the 1920s to the 1950s. Here's one:
If you can't visit it, there's an online gallery.
In D.C., the Smithsonian Natural History Museum has an exhibit about how the Natural History Museum has changed over the last 100 years. The exhibits have changed a little since these days:
If you can't visit, there's a pretty cool interactive before-and-after online feature too.