Friends Community School -- we've been doing these for years, and we've been happy. Expensive, and no pool, but lots of outdoor time and plenty of cool themes.
Greenbelt Recreation Center camps - great, affordable options, two week intervals and the KinderCamp for preschoolers has been terrific for both my kids. The director, Teresa is a gem.
Gymkana at University of Maryland (gymnastics camp - my daughter has loved it, but outgrew it recently)
TerpQuest at University of Maryland (haven't tried, but heard great reviews, we may give it a shot this summer)