Agree on Phi Phi Islands. Also look at the Similan islands, Phang Nga Bay (with the cliche James Bond Island) There's also lots of dive sites, Thai cooking schools, watch a Thai Boxing match, or go on an Elephant, 'safari.' (Just don't pick one of the shifty outfits!)
Sam Thank you, Stefan! Now that you mention Thai boxing match, do you know about how one could participate? I have a mate who would like to!