I head the aquarium is good the a must to see the island is the Conch Train. You can do it on your own which I recommend or usually through the cruiselines. I've never been there but a friend I am going with in Feb has been there and she said we must do this. Its a great way to see the island which is very small.
We'll be on the 12/15 Enchantment of the Seas, and making a stop here as well. We are looking at possibly going scuba diving here, although I've heard mixed reviews about diving here in December. We've been to Keys in December, and have always had great weather, but rough seas.
Walking around town is a lot of fun...it's a good place to rent bikes or go shopping. There are kiosks near the ship, as you exit, that offer parasailing and jet skis.
Also just returned from Key West via enchantment of the seas. We were here from 8am to 3pm so not much to do. They do have para sailing, shops, snorkeling, scuba diving or just shopping and eating at the many restaurant/bars. We took the train/trolley tour in the morning just so we could get an idea of what was on the island. The tour took about an hour.
I was there in October 2000 and am planning a trip back this October. We did the conch train right from the ship...about an hour tour and gives you a quick tour of the island and points out all the highlights. Duval Street is great (Sloppy Joes & the Key Lime Pie factory 2 must sees). If you plan on going parasailing, book it as soon as you get off the ship or before you go. When we were there, there were 2 ships in port and there are only 6 parasailing "companies" they were full by the time we got there. Also if you want to do jet skiing, take a credit card with you-they require one before you ride.
Have a great time, no matter what you do it'll be great!