I went to Belize a few years ago. We stayed at Ramon's on Ambergris Caye. It is a beautiful place and there is a lot to do there in terms of water sports. Snorkeling, sunset cruises, etc. We took a boat tour to some Indian ruins where we hiked and climbed up a temple. That was really neat. We also rented bikes and just biked around the island one day. That was fun as we found this little bar way off the beaten path. The snorkeling and diving there is awesome!
I have to say that it was not one of the better trips I've personally been on though. It was windy there at the time of year we went. I had a lot of trouble with motion sickness on the boat tours we took as well as the little puddle jumper plane we took to get a few places. I spent a lot of the time sick. :-( Also if we got much away from the resort area, the living conditions of the islanders kind of depressed me. But we took that trip probably four years ago and I suspect a lot has changed there since then.
Where are you staying? Hope you enjoy it!