There are things to do but its a matter of what you want to do as typically Key West is livelier at night. Things you might consider:
1. A simple day on one of the beaches. Now keep in mind these are not great beaches here so it may or may not be your best option on a short day. SMATHERS BEACH is the public beach or FT ZACHARY TAYLOR is the state park with a small fee.
2. Stop by the SOUTHERNMOST POINT in the USA a concrete buoy marking it as 90 miles from Cuba. Its several blocks down from the Hemingway House which is now a museum tour.
3. Do some shopping at the numerous shops.
4. Stop by IRISH KEVIN"S or SLOPPY JOES, my favorite bar for some lively music and a drink.
5. A snorkel trip either via the ship or on your own.
Key West is a nice little place to visit but when its a short visit it limits some of your options. Personally if I came in on a cruise ship I would hit the Southenmost Point, do Hemingway if you like history and if not walk and stop and shop and end up at Sloppy Joe's.