There are some ski resorts out in western Md/Pa (Wisp, Whitetail, a few others I can't recall). I've considered finding out whether they offer cross-country or snow-shoeing, but haven't looked into it yet.

And, yeah, there are tons of places to ride in/around DC. Peruse through this and get ready to pedal: BikeWashington.org

Also, I don't know if you saw the ride reports from our TE-DC Monument Tour last month, but we talked about doing a similar ride again after Labor Day. You'll definitely have to join us!