Note that there are two sites for the Air and Space Museum. Haven't been there in a while, but the big one our at Chantilly,VA, close to Dulles, has big parking areas and should be no problem. However, the original museum downtown, along with all the rest of the Smithsonian sites, is basically impossible to get to except on foot. Personally, I prefer the big site out at Chantilly. Plan on a FULL day to start seeing it. We spent two full days, one for a fast walkthrough, and the second for a detail view and what we missed the first day. If you can get into Cherry Hill campground, from there the subway will get you downtown.