When I visited Washington DC I had fun comparing it to locations in Fallout 3. The metro stations have the same names. There are a lot of unique subtle differences with a few of the buildings and major differences with others. The Washington Monument in the game, for instance, has a single hallway leading to the elevator with a nook off to one side for a statue of Washington. In the real monument, the hallway wraps around one side of the elevator shaft. There's a stairwell for emergency purposes, as well. And it has a two story observation deck up top. The elevator, which opens on both decks, opens from both sides as well. However, it's about as cramped up there as it is in the game. There is also a nook for a statue of Washington on the ground level in real life as well.
The Jefferson Memorial is vastly different, however. For one thing, in the game the rotunda is an enclosed interior accessible via entrances at ground level, but which enter in to the rotunda below ground. So in the game you actually have to travel down an interior in order to enter the rotunda. The real memorial's rotunda is open on four sides, accessible from exterior steps, and built way above ground. The real life statue of Jefferson in the center of the rotunda is also much bigger than it is in the game.