About a week ago I went down to the Steven Udvar-Hazy Center, which is part of the National Air & Space Museum. It's another building separate from the Museum located on the National Mall. It's a fairly new addition to the National Air & Space Museum located in Chantilly, VA near Dulles Airport. It was well worth the day I spent down there. They have some interesting and famous planes there, most notably the Enola Gay. There is also a Space Wing that has a lot of interesting things on display from the early days of the space program. If you don't like any of that, they did film part of Transformers 2 there! They also have an IMAX theater, tours, and flight simulators. I'm not a huge aviation person, but I found it quite an interesting, informative, and fun experience. I took pictures you can look through in the gallery below.
If you are interested in going I would recommend going early and leaving early. I l left around 2, mainly to avoid any traffic down there. Also I would pack a lunch or bring some sort of snack. There is a restaurant there, but it's a McDonalds. I personally try not to eat there, but if you like McD's you will be happy. It's a Smithsonian Museum so admission is Free. Parking is $15 though, so the best thing to do is get a minivan or large SUV and find a bunch of people who want to go!
For more information visit: http://www.nasm.si.edu/museum/udvarhazy/?hp=m