National Museum of the U.S. Air Force
This sprawling expanse of a museum is a wonder unto itself. It includes several entire hangers of complete aircraft spread over several acres (and has an additional off-site hanger of previous Air Force One aircraft). It traces the history of the USAF from its earliest attempts at flight to the jet age to space exploration - and even a display of the evolution of ICBM's (intercontinental ballistic missiles) from the Cold War. Numerous captured aircraft are included too.
This museum is so large and vast that it will take you an entire day to just walk through it and gaze upon most of the displays. It will take you much longer if you like to dwell and actually read about them.
It's a great way to spend the day with any aviation or military history buff, slightly older children, or any veteran.