2 islands to visit is ko samui & ko pang yang; stay in the beach huts on the beach
go to the full moon party
go elephant treking
have a proper thai massage (not the sort your probably thinking off!)
dont be afraid to eat from the streets; 20p will buy you a decent meal
go to the palace and hire a tour guide
go shopping - custom suits can be made for peanuts
dont buy DVD's, they are shit quality
hope that helps