Tumblers Today in the Sewer by the ammunition stash (hidden in a broken out wall in south east dead-end tunnel).
At the scaffolding at corner of 17th and Pennsylvania, there is a dead merc on the side with the leaning/bent light pole (you must walk up the pole to reach the merc) with a note (Pennsylvania Ave explosives note) about planted mines and activation switch location. The switch is past the dead merc. When you activate the switch, all the explosives go off in a chain reaction in the street.
The streets in this neighborhood appear to have been mis-numbered in the game, as 17th Street is west of the White House. The street marked as 17th Street in the game should probably be 12th Street.
The White House Plaza is already in the main game but is not marked on your Pip-Boy until you have installed Broken Steel.
Next to Metro Central is "The Clarabella", a very large and unique building.