Just back from a 3 nite stay at the Marriott Fairview Park, just off the 495 beltway,near hiway 10 from DC. About 9 - 10 miles from the government center in DC.Has all you ask for, except walking distance from Metro, although they do have a free shuttle to and from the station.
Right now, they are running a $59/nite weekend rate, which we got, Sat thru Monday. Nice place, good deal. Free parking, outdoor or indoor, your choice.