I have the rest of the day off. It's pretty nice out. I'm in DC...what should I do?

I’ve lived here forever and don’t even know the best museums, I haven’t really been to any of them since I was a kid. I know the Hirschorn is awesome and so is the Zoo, but what else is there? Maybe something that isn’t a “big-name” like the Air and Space or the Natural History. Free is preferred. Thanks all.