I did your excursions in Costa Maya and Cozumel in April and they were great. Mexican Cuisine you get to cook lunch and the consensus from all of us who did was it was way better than that night's dinner on board.
Grand Cayman-Stingray and Snorkel with Captian Marvins
Cozumel-not sure yet, maybe just wander around
Costa Maya-Tropicante sounds great, I'll check with my group and email him tonight, hope there is still room
Castaway- 5K and wandering the island