Greenbelt campground, part of the National Park Service, is in Greenbelt, MD, just 12 miles from the White House. The metro stop is only about 1 1/2 miles hike from Loop D, or you can drive to the station within minutes. Parking at the metro station is free on weekends, and $5.10 during the week. I stayed in Loop D, and it's a very quiet campground. Every once in a while, I could hear a siren, but that was only a few times during the week. It's hard to believe this oasis is so close to a metropolitan area. It cost $16 a night, or $8 if you have an Access pass. The campgrounds were safe, and had free hot showers.