You can hardly go wrong just walking the Mall either from the Washington Monument toward the Lincoln Memorial (and the nearby VietNam wall). Plus there are a number of federal buildings along the nearby streets: Supreme Court, White House, Smithsonian. I like the train station, not too far from the Capitol building, which is itself quite a photo op. Pick up a quick guide book and study it in your hotel or wherever you're staying. All of Washington is a photo op!