Our first port was Key West. We spent the day sightseeing using the Trolley for transportation. The Trolley was a good experience. We went to the top of the lighthouse which has a great view of the island, had fish-of-the-day at a local restaurant, ate key lime pie on a stick (over-rated), and generally enjoyed the warm weather. There were four of us from Wyoming and we had frost on the ground the morning we left for the cruise -- and we came back to snow. So my comments are obviously skewed.
Our second port was Grand Cayman. We booked a cheap excursion (by Carnival standards) to Tiki Beach on seven mile beach. We booked through Carnival simply for convenience. My wife, who doesn't swim, wanted to try snorkeling, and there is a reef right off shore where beginners like us can be in shallow water. We saw colorful fish. This worked well for us as beginners. We learned a couple years ago in Belize that you don't just throw people from Wyoming into the sea. They talked my wife into snorkeling from the boat rather than the beach and they were more persuasive than I was. Belize was a short snorkel attempt.
Our third port was Jamaica. We booked the Cool Runnings excursion to Dunn's River Falls. It was an excellent excursion. We climbed the falls which took about 45 minutes. The guides are experienced and keep people moving. It wasn't too difficult (we are all in our 50s) and quite fun. I read about aggressive vendors, but we didn't encounter any vendors there at all. We were there about 10am which might be too early for island life. We walked to a shopping area right off the terminal where we encountered a lot people selling lots of stuff. If your shopping experience is primarily US-based, then vendors in Mexico or Jamaica seem quite aggressive, which they are. We just say "no" and move on.