Bullfeathers is close to the Capitol, and is reasonable. Every other place I (personally) know of is either Mexican or a bar.
Whoever suggested the night tour of the monuments was spot on - they look so different, almost majestic, at night. There are a lot of ghost tours, too, in both DC & Old Town Alexandria.
If you have a car and chance to travel outside DC, consider Annapolis, Maryland, if you've never been. The City Dock water front and cobblestone streets are beautiful, and you can take a guided tour of the U.S. Naval Academy most days. You can get crabs (Maryland blue crabs) at Cantler's or Buddy's, and there are several pub style restaurants downtown.