The Washington Monument is the defining element of the National Mall's skyline: a 555-foot marble obelisk at the centre of the city's plan, aligned with the White House to the north and the Jefferson Memorial to the south. When it was completed in 1884, it was the tallest structure in the world. It held that record for five years until the Eiffel Tower went up.
The interior has an elevator to the observation level at 500 feet, with views across the city in all directions. Timed entry passes are required and sell out quickly — book well in advance at recreation.gov. The monument's exterior is particularly striking at night when it is lit, and the base is surrounded by a ring of 50 American flags, one for each state.