We have not been to any of these ports and have never had any interest in a Caribbean cruise, but the price was so attractive that we decided it would be a relaxing vacation as winter starts to grip Canada. I can tell you what we are doing which may help you think about possibilities.
Costa Maya: There are a number of well-reviewed beach clubs fairly close to the port, but we decided on a more remote one 20-30 minutes south that is quieter and has snorkeling off the beach. Almaplena Eco Beach Resort, http://www.almaplenabeachresort.com/
Harvest Caye, Belize is an island owned by NCL with beaches, pools, restaurants, etc. From the videos, it looks nice enough if rather sterile. There is a ferry that goes to Placencia on the mainland, which is what we plan to do and explore on our own for a while.
Roatan, Honduras: We have booked a tour through Rony's Tours, http://ronystours.com/. We chose to do a private, free-style tour, but there are lots of options.
Cozumel: Again, we booked a beach club based on Trip Advisor and Cruise Critic reviews, Nachi Cocom, https://www.cozumelnachicocom.com/ for a relaxing day on the beach.
None of this is very exciting or enlightening or mind-expanding, but that's not the point of this cruise. And frankly it will be a refreshing change after too many cathedrals and quaint cobblestone streets in Europe.