The Little White House was originally a naval officer's quarters, built in 1890. Truman discovered it during a recuperative stay in 1946 and returned eleven times during his presidency. The house is now a museum run by the state of Florida, with guided tours explaining Truman's presidency and the decisions made here.
The tour is better than you might expect: the guides are well-briefed and the house retains the scale and intimacy that made it a productive retreat. The garden and the surrounding historic naval station compound are pleasant to walk. Combined with the Hemingway House, the two historic homes give a good picture of Key West's appeal to significant Americans.