Hi Purpleally, I have been to these ports and done these excursions.

Cozumel - We went to Discover Mexico on our own before free excursions. It took us on a guided walk through miniature reproductions of famous Mexican landmarks. I really enjoyed it.

Belize City - We signed up for the Absolute Paradise but due to high winds, we had to change to the Airboat Adventure and Belize City Tour. The city tour was short and little of interest, but the airboat ride was fun and we got to see some wildlife.

Santo Tomas de Castilla, Guatemala - The excursion we took was the Countryside and Rio Dulce By Land. This took us into the countryside to a huge lake. We rode around, stopped at an Inn for refreshments and then again at Castle of San Felipe. This was quite interesting. The port in Santo Tomas has great shops, local artists with friendly people.

Key West - Easy to walk around on your own. I have taken the Conch Train, which was a fun way to see the sights. Also the Aquarium, which is old, has a nice grouping of animals. The Mel Fisher Maritime Museum is worth a look.