Explore the Countryside
Not far outside of Washington, DC, this is a great place for a day trip to escape the bustle of the capital. The park itself is beautiful; large open fields, remnants of fences and other battlefield installations, and lots of informational plaques and monuments as part of a walking tour. Very informative, while also leaving plenty of time to explore the area or enjoy a picnic.
Speaking of which, don't miss the nearby Winery at Bull Run for a relaxing wine tasting, and downtown Manassas has a lot of great places to eat!