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