National Air and Space Museum D.C.

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There are some great planeterium shows for kids here, with Sesame Street characters and others. There is plenty to see on any given day including climbing on board planes of different eras. And of course, it's free!
http://www.nasm.si.edu/


Related activities include the National Air and Space Museum Hangar in Dulles (same link) and the Maryland Aviation Museum in College Park http://www.marylandaviationmuseum.org/