This is a picture of the flagship building of the Smithsonian National Air and Space Museum. There are two other nearby facilities that are part of the National Air and Space Museum:
The National Air and Space Museum focuses mostly on aeronautical and space equipment, being more of interest from an engineering perspective than from a purely scientific ones. Still, even the most geeky of pure scientists will be hard pressed to be stoic in the face of actual capsules that have traveled into outer space and to the moon, and kids will absolutely love it. I still remember the exhibits here from when I went to D.C. in Middle School, so I'd definitely include it in any list of sites to visit in Washington, if you can fit it into the schedule.