One of the best sites for a view of DC is The Old Post Office which gives you a 360 degree view of the District. Besides the view, you get a tour of the Clock Tower. The Old Post Office Clock Tower houses the official United States Bells of Congress which were a bicentennial gift from England celebrating the end of the Revolutionary War. The Bells of Congress ring every Thursday evening and on special occasions. The over 100 year old building is being converted into a luxury hotel by Donald Trump, and not a moment too soon.
Other than the trip to the top for the view, there is nothing else of interest. There is a very average food court on the base floor which I would suggest you avoid. Depending on the lines for the small elevators to the top, it is about a one hour diversion during your day to go to the top, take some pictures, and exit the building.
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