Though previously recommended to me, I stumbled across this place just outside the Federal Triangle Metro Stop on the backside of Trump International Hotel. I actually stopped for coffee, originally, but upon learning the clocktower was there, I stuck around to finish my coffee (can't take it with you inside) and then walked down a hallway museum of the history of overall DC planning and some of the Post Office History, as the hotel was the US Postal Service's original headquarters. I took the elevator up to the top and got an amazing view of the city. I think it was the ... third highest spot in DC? Granted, the Washington Memorial's been shut down for years since a big earthquake shook it up. You can see most of the flat city area surrounded by mountain ridges (who knew?) and some of the Potomac. If you're looking to get a great city tour, this should be included!