Koh Chang, Thailand.
Stayed there for 19 days once. Absolutely loved it.
This island of the east is very photogenic, quiet and peaceful. In the center of the island it is quite untouched so if you are into birdwatching or other island rainforest activity you are going to enjoy it there. It is a much quieter place than many other thai tourist destinations but that was what I liked about the place the most.
Go there and buy a hammock, listen to the waves and eat delicious thai food.