We just returned and did two trips that we especially enjoyed. The first was a sailing/snorkling trip on Next Wave. The captain, Rory, has circumnavigated the world on a boat that he built and is quite knowledgeable. He will bring a local snorkling guide. We met both Edmund and Tony who were terrific. Next Wave also does sunset cruises and charter trips. You can book directly through e-mail or at the Purple Space Monkey. Several resorts do bookings as well, but you probably get a better price doing it directly.
The other trip that I highly recommend is a kyaking trip down South Stann Creek with Greg or Julian Chun, Mayan guides who live in Maya Center. You begin in the jaguar preserve and finish up about four hours later on the Southern Highway. We saw amazing wildlife on the trip and had a great time on (and mistakenly in) the water. They provide kyaks and lunch and a wealth of experience. You can reach them through their website at Mayacenter.com or by calling them there. They do hiking trips as well and are very knowledgeable and personable.