The Blue Hole trip for snorkeling (not diving) & it helps to make friends @ the dive shops...
As far as eating and drinking...I have a tendency to eat much lighter when I am in Belize. From the Great White North, I am, so the humidity has a tendency to take its toll on me and eating less food & lighter foods helps me handle the humidity better--plus drinking tons of water helps too.
Don't get me wrong, I drank my fair share of Belikin as well as One Barrel during my stay.