While not a day trip in and of itself, the DC Post Office Bell Tower is worth a visit next time you’re in Washington.
Tours of the bell tower at the Old Post Office Building in DC (Federal Triangle Metro) are FREE and you also get you a great view of Washington. There’s a food court in the building, and you’re within walking distance of the museums.
From the website:
When completed in 1899, the Old Post Office Building was criticized as looking like “a cross between a cathedral and a cotton mill” by the New York Times. The criticism sparked a seven-decade struggle to save the building. Rededicated in 1983, the Old Post Office is an example of the remarkable transformation old buildings can undergo and the rich possibilities they can offer.
Tours of the Old Post Office Tower provide visitors with the history of the building, a breathtaking vista from the 315-foot clock tower and a view of the Congress Bells. Tours begin at the glass-enclosed elevator on the stage level.