First things to know: it's one of 2 museums, the other one is by the IAD airport and contains the bigger stuff that doesn't fit there (and couldn't have been brought there through the streets for some bits). This museum takes more than a full day to visit if you go through the exhibits, I'm efficient but I had to go back twice to finish it (mostly), and that was without even going through the planetarium or IMAX, but I did do the 1H guided tour once and that was worth it.
Then, you need at least another full day to see the much bigger Smithsonian Air and Space Museum by Washington Dulles (IAD)
google has the copy, they have the real one :)
US and Russian capsule rendez-vous
X-15, the first prototype to fly to space before rockets
skylab, before the space station
Howard Hughes' speed record plane
The actual voyager plane that went around the world
great way to show how a modern telescope works
These few pictures do not give it justice, the amount of history of aviation, space, including what happend to other countries, how GPS works and why, how the moon landings were done, and how we're going to Mars now, it's a lot of information.
I guess that's expected, but it is indeed bar none the best such museum I've seen in the world so far.