Koh Tao
Koh Tao is off the west coast in the Gulf of Thailand. Beautiful beaches to relax, great diving, nice main area with good restaurants and bars. The town has a really nice chill vibe to it making it a good place to stay for a while.
Koh Phi Phi
Koh Phi Phi is off the east coast in the Andaman Sea. This island is famous from the movie The Beach. There are picture perfect beaches but you have to search a bit to get away from the crowds. Best option is to rent a kayak for the day – just watch out for the thieving monkeys! Its also a huge party island with cocktail stands ever few feet.