This is my favorite thing about flying back into DC from a trip: the famous River Visual Approach to DCA, the path planes must take to avoid no-fly zones when landing from the North in clear weather. Sit on the left side of the plane during a river visual landing, and you get great views, from the Mormon Temple and the Beltway, down to the Cathedral, right down to Georgetown, the Watergate, the National Mall, Hains Point, and Southwest Waterfront.
When flying in to DC with a chance to reserve seats, pick a window on the left side. Even if you end up doing the less climactic approach from the South, you get a great view of Alexandria and the George Washington Masonic Memorial before touching down. (Conversely, if flying out, get a window seat on the right side of the plane for similar views.)
Bigger video after the jump: