For inland tours, we recommend local tour operator: Truly Wild Belize. The proprietor, Jason, has packages that include zip-lining, horseback riding, Mayan ruins, cave tubing, chocolate farm, Jaguar Reserve, and others. Truly Wild also has a shuttle service to Belize City & elsewhere. Combining a shuttle from the airport with a tour is a great way to save money and break up the 3 1/2 hour trip by seeing some local points of interest.
If you're looking for a relaxing snorkel trip with some sun-bathing mixed in, consider Daytripper. This is a good 1/2 day sail on a catamaran for less than $100 each. It includes a stop or two to snorkel off the boat, lunch, and rum punch.
Authentic Fishing/Snorkeling and more...
For an authentic experience on the water, there is no better than Eric "Reco" Foreman. He was born and raised in Placencia (he and his family live just across the lagoon from Hibiscus Beach) and knows the waters, reef, and cayes as well as anyone. He will combine fishing and snorkeling in one trip, or just take you to a secluded beach to relax and swim off the sand. He often does over-night camps on islands & cayes and kayaking trips. If you are looking for a more leisurely float, ask Reco about a Lagoon Cruise on his pontoon (this is a great option when the sea may prove too rough).