We will be on the Legend a few weeks after you. We are in the process of do the same, deciding on excursions, and have to say that nothing jumps out at us as must do. But for us, that is mostly due to having done this ship, and most stops before. If we had not already done them, the top excursion of the whole cruise would be Sting Ray City.
Unfortunately that doesn't narrow it down much, as there are about 10 excursions that take you there. If you are going there, you need to decide if you really want to make a day of it, and to Hell and the Turtle Farm too. Our friends will be doing that. We are still thinking about it. Repeating Sting Ray City is worth the money, but Hell sounds underwhelming and we're not much interested in the turtles (even if they are the national symbol of Grand Cayman.)
Beyond the addition of the turtles and Hell, the other differences in all those Sting ray tours are mostly in the kind of boat that gets you there, and whether or not they stopsomeplace else. You'll have to read through the descriptions.
In Cozumel, the must do excursion is the Mayan ruins of Tulum. If you like archeology this is your chance to see a real pyramid. But if ruins are not your thing, it's a long hot day. The bus ride is over an hour, and you have to take the ferry to get there. Beyond that, there are tons of fun things to do in Cozumel: Chanchanaab park, several beaches to choose from, Best reef snorkeling we ever did was on an excursion there (for better swimmers). There's zip lining, and jeep excursions (must drive a stick) and four by fours. But if you want to take it easy, Cozumel is the best shopping port on this itinerary, and lunch at Pancho's Backyard is practically a tradition for us. Best quacamole and Margaritas ever. (Pretty good shopping in G. Cayman too, but everything there is more expensive.)
In Belize, if you have no issues with walking, do the river tubing. I've always wanted to do that, but last time hubby had just had a hip replacement, and now I've got issues with arthritic feet and a possible torn tendon that is taking months to heel. Who knows how it will be in February. (You have to walk for 30 minutes carrying your inner-tube to get to the river from the bus.) Otherwise, it looks like the best excursion there. Besides that Belize has some of the very best snorkeling, and diving if you dive.
Mahogany Bay, Well there is a beach area right where the ship docks. Some say it's nice, others bemoan the fact that it is man made and commercial. I've heard wonderful things about Tabayana Beach, and that's where we'd go, except for how short of a day we have there. It doesn't seem worth the money and the 45 minute ride, for such a short time.
Definitely Sting Ray City in Grand Cayman. In Belize we have taken a tour to the Mayan ruins of Lamanai that was very good. It features a 26 mile boat ride through Mangrove forest to the site. Tulum in Cozumel is interesting due to it's seaside location but it is relatively small. Cozumel is mainly shopping. I would go along with Cruizer and recommend Chankanaab Park.
We did the Relax on the Beach in Roatan for an easy beach day and great price. In GC, I recommend the turtle farm - buy your tickets through the farm's web site, though, to save money. They put you on the same shuttle as the ship excursion groups, but you get to wander on your own for as long as you like without being stuck in a large group. We also LOVED the dolphin encounter in Coz. Again, book independently for the best deal and smaller group.
Belize is not a port I care much about, so I can't recommend anything there. All excursions seem to require an over-crowded perilous hot bus ride across rocky terrain to a good length of time. I'm sure there are some exceptions, but for me, I'd rather stay on the ship.