I will second all of Rex's choices.
NASM is an incredible place - I've been there about a dozen times - never gets old for me but my family on the other hand....
While in DC
The Paul Garber Facility (is that right?) is a great place.
It's the NASM restoration facilty. You have to call ahead and schedule an appointment, but it's worth the trouble.
When I visited they were well into the N28 restoration. Had an interesting conversation with the lady who does all the rib stiching. Beautiful work, hope she found an aprentice.
They had to drag me out
Also in progres was a hawker Hurricane and a Japanes Serian (sic) an odd plane that was meant to be carried in a sub.
Also check out the Museum of American History (part of the Smithsonian) Steam engines, farm equipment, all kinds of interesting stuff. Nice models as well - the displays on bridges I found to be particularly interesting.
DC is a great town for museums
Of course The Old Rhinebeck Aerodrome is a favorite. I only live about 1.5 hours away so i get to go at least one a year. Check out the town as well. The area was depressed during the 50s so they didn't have enough money to screw up the architecture like everybody else.
Very pretty.
Mark