My tips on these ports:
Key West: Plenty of activites/excursions you can sign up to do when you get off the ship. My opinion this is not a port you need to book an excursion on the ship for. This is not a beach port. This is a port where you can rent waverunners, go on a shark excursion,party boat snorkeling etc...all right there on the pier...you can ride the Conch Train, rent bikes or scooters. We chose to follow the map the ship gave us as we departed....and the sign at the end of the street that says to Duval Street. We turn right on Duval walk to the end there is a butterfly conservatory you can visit, or go to the end of the street turn right and take pictures at the "Southernmost Tip of the US"
If you head northon Duval the Aquarium is at the north end of the street. Plenty of interesting shops etc.