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.