Kathy,
Bummer about being changed from Belize to Costa Maya. We went cave tubing in Belize and loved it. (we took our own excursion instead of the ship's and were able to tube through two caves instead of one.)
In Costa Maya, we brought our own snorkeling gear and took a taxi into Mahahual. We walked to the other end of the town and found a nice beach with another couple from the ship sitting there. The owner of the food/beverage stand across the street served us margaritas and chips/salsa. We snorkeled, ate, read books, and sat in the sun, and visited with our new friends. The farther you walk away from the taxi stand, the town gets poorer. You could see the ship from the beach we were at, but it is not walking distance.
I wish now I could have read the post about the schools. I never thought to bring school supplies...what a great idea. The website and pictures are great. I will definatley do that next time we dock there.