This is very off the beaten path if you are staying in DC with no car. It is accessible by public transportation, and if there is not an article posted on this I will write one.
This is an awesome facility, and having it in a hangar, with a control tower sets the mood. There are unbelievable things here...the Enola Gay, the shuttle Discovery, a blackbird, and many other aviation, engineering and historical marvels. For those into wartime aviation, they have a collection of Japanese, German and Soviet aircraft, many of which are the only surviving examples. Add to that many spacecraft, helicopters, human powered flying machines....it just goes on and on.
It is an OK place for kids, but probably a quicker visit as patience will not allow reading of all of the explanations and history.
Like all Smithsonian facilities, admission is free.