Come around the full moon in May to see the whale sharks. We do whale shark trips from here at Thatch Caye during the full moons of March, April, and May. That is when the snappers are spawning and the whale sharks predictably come to filter feed in a particular spot that the snappers go to called Gladden's Spit. We are about an hour boat ride from there and do the ride right down the inside of the reef the whole way so it isn't that bumpy like it is coming from Placencia. As Steve suggests in a previous note, do some diving in San Pedro and then head south so you get the full experience that Belize has to offer.
Thatch Caye does inland tours to Mayan ruins, waterfalls, cave tubing, Garifuna and Maya dances, herbal/medicine trails, butterfly farms, etc. and has excellent snorkeling spots all up and down the reef nearby for non divers as well.