DC has one of the best bicycling clubs, as well as a few good kayaking clubs (both white water and flat).
The Northern Virginia Photographic Society is worth looking into. There is also the McLean Camera Club, a club in Sterling, and there might be one in Leesburg.
Try DCShutterbugs web site for a listing of local clubs and "meet-up" groups (Dead Link Removed). I belong to the Bowie-Crofton Camera Club. We have just started our new year and meet every Monday night (http://www.b-ccc.org)