I heard from several people with excellent ideas to "must see" places in the D.C. area. Here they are--thanks, Alyssa, Nick, Mary Mac and J.C.!
1) Bluemont Vineyard--on a clear day you can see the Washington Monument, on an cloudy day you can just drink.
2) The Lincoln Memorial and Korean War Memorial late at night. Really cool looking, and a much different vibe than during the day.
3) Middleburg. Good food and you can pretend to be fancy. If it was good enough for Jackie O, it is good enough for me.
4) Great Falls and/or Prince William Forest Park. Good hiking!
Since you mentioned Williamsburg, I would suggest the Virginia State Capitol building in Richmond. I have not been since they opened the new visitors center, but it is a beautiful Jefferson inspired building.
Mary and J. C. McElveen:
The Jefferson Hotel and Virginia Historical Society in Richmond, Monticello (of course!--DV), Congressional Cemetery in D.C.
Excellent additions, blog people. Keep them coming!