I hope that this is the first of several entries for me. I highly recommend Railey Beach in Southern Thailand. Although not an island, you can only get there by long tail boat from Krabi (also worth seeing!).  It has some of the softest sand I have encountered and the days will just melt into each other while you are there. Rock climbing, snorkeling, kayaking, and fresh fish make every day both rich and relaxing. Dont miss.