While in Key West, why not check out Ernest Hemingway's house? Then see his six toed cats.
Check out the bars and hot spots along Duval Street.
There is a land marker there at the end of one street that says "Southernmost Point in the USA"
Maybe you can stop by and see it and get your picture taken in front of it?
Make sure to get a key lime pie, or the other thing Dave mentioned, or key lime juice.
Check out Jimmy Buffet's Margaritaville. Which if I remeber is allot like Hard Rock, Planet Hollywood, or similar restaurants.
That is all I can think of at the moment. Have a good time on your cruise