Just peaking at 555ft (and 5in), the Washington Monument is the tallest building in the district. It took two phases of construction to complete; note the different hues of the stone. A 70-second elevator ride whisks you to the observation deck for the city's best views. Same-day tickets for a timed entrance are available at the kiosk by the monument. Arrive early.
During peak season it's a good idea to reserve tickets in advance by phone or online ($1.50 fee, plus additional $3 if you order more than 10 days prior).