Key West, Florida: We were here once in 2000. Other than the beautiful sunsets at Mallory Square, the most popular thing to do is taking a walk down Duval St, where you will find watering holes such as Sloppy Joes, and the first Jimmy Buffett's Margaritaville. We stopped in for a "Cheeseburger in Paradise", and if you are a Parrot Head, you will find other landmarks such as Caroline St. Just make sure you don't overindulge, thereby extending your stay at Key West (if you miss your ship).
Some say as you walk around Key West, you might even bump into the ghost of Ernest Hemingway.
If you are lucky and your cruise ship stays until after sunset, there are musicians, jugglers, fire-eaters, and all sorts of street performers that entertain you at Mallory Square (adjacent to the cruise ship docks).