For my money, this is the closest thing to a postcard-perfect paradise as there is in this world. Happy little clouds (-Bob Ross), blue-green ocean, mammoth limestone peaks, longtail boats swaying in the breeze, and an unending tropical sandscape.
The island of Koh Ngai is a short boat ride away from Koh Lanta, Ngai's more famous big brother. Ngai features a few restaurants and hotels set off from the beach, but it is, by and large, a throwback South East Asian hideaway - sleepy, relaxed, and unblemished by ferocious corporate development (re: there are no 7/11 shops here). If you make it out to Koh Lanta, do yourself a favour and book a day or two on Koh Ngai - but only if you're interested in chilling on the beach, drinking juice from raw coconuts, and shooting pretty pictures.