The Udvar-Hazy Museum is one of the finest collections of air and spacecraft in the world. Among the hundreds of exhibits are the Enola Gay, the space shuttle Enterprise and the Concorde SST. There's even a small scale spaceship from "Close Encounters of the Third Kind."
If you go:
Free, but public parking is @ $15 per car. Plan at least 3 hours. There is a food court on the premises.
Links:

14390 Air and Space Museum Parkway
Chantilly, VA 20151 USA
info@si.edu
Phone Numbers:
National Air and Space Museum
+1 (202) 202-1000
+1 (202) 202-1000