Old Post Office Pavilion
Completed in 1899, this building was the main post office in DC until 1914. The structure is the third tallest in the DC, and it has an observation deck in the tower that is managed by the National Park Service.
The Old Post Office is also home to the official bells of Congress. They are replicas of the bells at Westminster Abbey and were a bicentennial gift to the USA from the Ditchley Foundation, a group that aims to promote good relations between the USA and the UK. They are rung on special occasions and federal holidays.