We just did the same itinerary on the Voyager. It is hard to make recommendations when we do not know what your interests are - but - this is what we did.
Belize - Coral Breeze snorkel excursion - their web site is www.coralbreezelimited.com. We had a great day with them. The snorkel stop with the stingrays was very similar to the one in Grand Cayman but without the crowds. The snorkel stop was pretty good but not as nice as in Grand Cayman. We really enjoyed our time on Caye Caulker and would love to go back there and spend more time.
Costa Maya - We walked to the little fishing village of Mahahual and relaxed on the beautiful beach for a couple of hours and then did some snorkeling at the beach by the pier. Not the best snorkeling but I did see some fish that I have never seen before. The village is great because it is very layed back (not like the area by the pier).
Cozumel - We spent most of our time shopping because my friends had some special things they were looking for but I would have loved to have spent some time at Paradise Beach. It is a new beach with no admission fee and almost everything is free except for food and drinks. No charge for chairs, floats, kayaks, snorkel gear, water trampoline, etc. It has been getting rave reviews. All you have to do is grab a taxi and go there - no excursion necessary.
Grand Cayman - We did the Stingray City snorkel trip with Dexter. There were 19 of us so we were able to charter his boat all to ourselves for $28/person. We snorkeled with the stingrays and then stopped at a reef for more snorkeling. It was a wonderful trip.
If you would like to see some pictures of these places I have to albums at http://community.webshots.com/user/jean_lynd
I also have two albums from our Explorer cruise and I took quite a few pictures around the ship that you might also enjoy.
I hope this helps!
