We do day trips to DC all the time.
Like Kevin mentioned, there's a Hampton Inn and a Holiday Inn Express (both with free breakfast!

) in Largo, MD and less than a mile from a Metro station. I'm guessing you would probably want to stay on the VA side, though, because then you could avoid having to drive around DC on the Beltway.
There are a ton of hotels in Arlington and Tysons Corner in VA--just about every chain imaginable. The hotel websites will usually tell you what Metro station they are nearest to. Both towns are very CROWDED and traffic can be an issue. Tysons Corner is a couple of miles away from the Vienna Metro station, but a lot of the hotels offer a free shuttle to/from the station--the websites will tell you.
You may want to drive to the station yourself. Parking is free at the Metro stations on weekends.

All of the stations can be found on Google Earth.
One thing you could do in VA is swing by Dulles Airport as well--there is a wing of the National Air & Space Museum located next door that houses some cool planes like the Concorde, SR-71 Blackbird and the Space Shuttle.
Anyway, I don't want to take up too much space here. Sounds like Fed can help you out, and if you want to PM me for other ideas, I'd be glad to help out too.