The Kei Islands are in Indonesia and are possibly the best place in Indonesia, if not the world, for a vacation on a 3 kilometer long beach with white sand, all to yourself!
The Kei Islands are located in South-Eastern Maluku. The best beach is Pasir Panjang, located on Kei Kecil Island.
Laze the days away reading your favorite books, organizing a snorkeling trip, finding somebody to give you a massage on the beach (not so easy on weekdays, where you will likely have the full 3km beach for yourself), fishing, going for long walks on the beach, or go for something a little more adventurous. You can explore caves, visit offshore islands, learn Maluku traditional cooking or take painting courses.
Don't expect party - it will be just you, your book, and three meals per day served in your full-board accomodation. There wouldn't be any other food option nearby, anyway.