Spend some time on the Smithsonian's website. They have so very much to see.
You mention Air & Space -- check out the extension that opened a few years ago near Dulles Airport.
Also, carve out 2 or 3 hours for the Natural History Museum. That's my fave.
And check out the IMAX films.
http://www.si.edu/museums/Once you get to DC, the best stuff is free.