Koh Sami is one of the most popular places in Thailand for a beach holiday. Even though the cheap accommodation of the past is on the decline, it is still possible to get a room of 250Baht.
Koh Samui provides all types of water sports, diving, snorkelling, windsurfing,etc. There is a good coast road around the aisland that provides access to all tourist facilities. I recommend to hire a motorbike - only £200Baht per day and go around the island. Always wear a helmet! You can visit the island around and you are independant.