Where except Cherry Hill,MD for DC?

Cherry Hill's big advantage is access to METRO (subway). There is a bus that takes you right to the station. Fairfax County in Virginia has some nice parks with campgrounds, but I think you have to drive to METRO and that means fighting the commuters for the limited parking at the stations Mon-Fri, so you'd need to start early. Lake Fairfax Park reportedly has a trail that leads to the Wiehle-Reston stop on METRO's new Silver Line, but it involves a bit of a walk through the neighborhood. Haven't been there myself. You might want to check with them, perhaps there are bus options, too, although I haven't seen that mentioned.

Getting to Cherry Hill from Leesburg isn't too difficult, assuming you don't mind the Beltway. Might not want to shuttle back and forth a lot, especially if making the trip during rush hour.