Traveling to Washington DC to see the Smithsonian Air & Space Museum we also spent 6 hours at the Steven F Udvar Hazy Center at the Dulles Airport. Now transport to the venue is limited and there is nowhere to store luggage so check into your hotel first and go back but it is worth every cent it costs to get there ( It is about $75 in a cab from Dupont Circle) or cheaper on the metro and takes about 1 hour 20 to get there on metro.
Everything form the Enola Gay to the Space Shuttle Discovery is on display. It is geek heaven. It is set out so you can see the planes from below and also above on walkways. Oh did i mention it is actually FREE so you only costs are transport and food (Macdonalds is the only on site food)
This was a highlight of our visit.