Koh Samui is always a good tip for a period of up to 10 days. Quite a few prefer to stay forever. The island is located near the islands of KoTao and KoPhangan. Island hopping is ideal here. There is a good selection of hotels. From hostel to luxury. Tip: It is very quiet here in the off-season.
Ko Samui is south of Koh Phangan. One of the most popular destinations in Thailand. Much quieter than Bangkok and so how tropical Thailand imagines.
The island is not particularly large. Within one afternoon you could circumnavigate Ko Samui nearly even with the scooter. This is what makes these islands so interesting. It comes down to a feeling of tropical island paradise.