Key West: Took the conch train tour, was good, but next time would try one of the other trolley tour's as they seem faster. Had lunch @ Bagatelle, EXTREMELY good seafood soup, and HUGE peel and eat shrimp. The Tortuga rum cake company has (FREE) samples that are delicious.
Belize City: Booked a cave tubing tour through Coral Breeze. Had a very nice time, but was somewhat rushed through the jungle and river due to waiting for late tenders. If you go to the cruise line and tell them you have a private tour booked, they will let you off early. Our tour was a better deal then the ships though, we had only 5 people in our group, versus hundreds in the cruise groups. Ivar our guide was very interesting and informative, as well as being helpful when we got stuck in shallow places!! The lunch at the end was delicious, authentic Belizian food.
Cozumel: Took a cab ($10 each way) to Chakanaab park. The park ($6) was BEAUTIFUL, had dolphin shows, snorkeling, sea lion shows and beautiful beaches to relax on, as well as nature trails and terrific PINA COLADA's SERVED IN PINEAPPLES. My 3 year old had a great time at beach on the lagoon.
Progresso: Took a cab ($50 for the day-- our driver waited there for us) to the Mayan ruins of Dzbultan and swam in the attached Cenote, it took about 30-45 mins to get there. There aren't really changing rooms, so if you plan to swim, wear your bathing suit. It was hot and the water was refreshing.
Overall the Carnival cruise was lots of fun. The room stewards are great as are the dining room waiters. It would be nice if the cruise was in port for about an hour or two longer, as port days were sort of rushed. If you have a young child, bring your own water wings as they don't sell any on board.