My wife and I camped at Greenbelt National Park in May of 2012. It is actually only minutes from where we live but we only had a couple of days and were itching to get out.
Greebelt is not a large park but it is big enough to provide good seclusion from it`s suburban surroundings. The campground is well situated in the middle of the park`s 75 year old forest so that nearby road noise is minimal. We found the bathrooms in fair condition but they could have been cleaned a little better or more often. The campsites are mostly wooded and for the most part, well spaced. Half of the camping loops were closed at the time we were there. Apparently their official season starts on Memorial Weekend and all the loops would be open then. I thought that was an oddly short season but like many National Parks these days, it seems they were suffering from budget cuts. They have a small nature center that is OK but a bit run down and there are several trails although one we hiked came too close to a neighboring HWY and was noisy.
Most people I suspect camp here as an inexpensive alternative to Hotels for Washington, DC sight-seeing. The fee is only $16 per night! We noticed that most of the occupants left early and returned late in the day. There is a Metro station (College Park) that is only about two miles away, providing an excellent way to go see the DC sights. If anyone wants to stay at Greenbelt for that reason, it is probably the best choice in the DC area for those who want to save money by camping.