Udvar-Hazy Center (Air & Space annex) is a real time commitment — fit it in only if it's a priority.
Allow Half day incl. travel.
For aviation fans this beats the Mall location — full-size aircraft you can walk under, Space Shuttle Discovery, an SR-71 Blackbird, all free. The honest catch: it's way out near Dulles Airport with no Metro, so it's a car or a long transit haul, and parking costs more than the (free) museum.