The holocaust museum is VERY depressing, the group of men I went with all cried. They have baby shoes from inside the concentration camps, etc. The smithsonian museum of natural history has a nice collection of dinosaurs and stuff in it which is pretty cool, and the smithsonian museum for air and space (or something like that) has a lot of neat stuff too if you're interested in that. (I wasn't at all, but it was still kind of cool.)
Also the smithsonians are all free, which is a nice tip
Be prepared to walk a lot!