Just 22 minutes from the hotel, the Steven F. Udvar-Hazy Center displays thousands of aviation and space artifacts (including the Space Shuttle Discovery, a Blackbird SR-71, and a Concorde) in 2 large hangers. The center opens at 10am and parking costs $15 (entry is free!). We recommend snagging some tickets online on their website before arrival.
Just 12 minutes from the hotel, the Manassas battlefield park has plenty of lovely trails and historical sites where the famous Battle of Bull Run took place.