In Key West, a visit to the Little Whitehouse, as Harry Truman called it, is highly recommended.. This is a very interesting place where Harry Truman (who had very little money to spend on a vacation home) spent his vacation time. It was a US Naval station that was converted to a private residence (at very little cost because it is such a modest place)for Truman. It is an important part of American history and takes about an hour and a half or so.
Also, walk down the main street to the end and see the southernmost point of the USA.
Hope that this helps.