not much is cheap in Belize, especially san pedro. it is the most expensive country in Central America.
Besides your partying at night, in the day you can snorkle ($25-30/ day), Windsurf ($$$$), swim, dive ($60/ 2 tank dive), fish ($150+ per day), hangout in the sun or under a palapa or hamock reading a book listening to music, go to the gym. There is only so much to do on an island. I am never board there but as far as keeping yourself busy and for cheap, that's a toughy!