As long as people are recommending DC: I'll 3rd that recommendation.
I'll also note that the two recommended parks:
Cherry Hill Park and
Greenbelt Park,
MD are only about 5 miles from each other! (The differences between them are quite dramatic: Cherry Hill Park is more of a "resort park" with full hookup sites, pools, theater, mini-golf, etc. and Greenbelt Park gives you more of a "roughing it in the woods" feel.)