Samui sits in the middle of an archipelago, and the best days out are boat trips rather than inland drives. Ang Thong Marine Park — forty-two limestone islands in a protected reserve — is the headline excursion, with kayaking, snorkelling, and the emerald saltwater lake on Koh Mae Ko that inspired The Beach. Koh Phangan, a half-hour ferry ride north, is where the monthly Full Moon Party happens at Haad Rin; go if that's your thing, or skip it and stay the day for the quieter beaches on the west side. Koh Tao, another hour on, is one of the world's cheapest places to get open-water scuba certified.