take a look at this thread. i thought this person did an extremely efficient 1-day trip and packed in quite a lot of stuff.

http://www.flyertalk.com/forum/showthread.php?t=610470

One other piece of advice. In case you are not aware, the 2nd "campus" of Air & Space is right next to IAD and a solid 45 min from the city (with no traffic). Might want to try to work it into your trip to / from IAD.

Also Spring, particularly early to mid April is high tourist season due to the Cherry Blossoms / Cherry blossome fesival. Might want to google to find out which days that is - hotels are expensive and hard to come by then.