6th and Independence Ave., SW, Washington DC, DC 20560 (National Mall)
First tip: Go early in the morning.
This was an amazing museum. Anything that you could think of relating to air and space was there. From the very beginnings of the thought of flight to looking out beyond our solar system. There is so much to see and discover. There are two floors and 10 or more separate sections of exhibits. There are airplanes suspended from the ceilings. There are satellites and rockets that had at one time actually been in use. There are many interactive exhibits and so much information that we could not see or read everything. There is a two-floor gift shop as well to purchase child-sized space suits, toy airplanes, science games, T-shirts, hats, postcards, books, etc.
It is a Smithsonian museum so it is open free to the public, but they do accept donations. It was a wonderful visit. If you see anything in Washington DC, this is one of the places to go.