Not sure of your kid's age or interest, but Capitol Hill Day School, Capitol Hill Arts Workshop (CHAW), Polite Piggies, various things at the Hill Center, horseback riding camp in Rock Creek Park, BETA academy martial arts at 14th and Florida, Breathing Space yoga, national building museum, national conservatory in SW (
http://www.nationalconservatoryofarts.org/ ...it's pretty new so I don't know much about it), lots of DPR camps in DC too though their quality can really vary.