A flat little island of coconut palms between Koh Chang and Koh Kood, with almost nothing on it.
Find a room before you go.
Koh Mak sits between Koh Chang and Koh Kood and is the flattest and most manageable of the three. Most of it is coconut and rubber plantation, owned for generations by a handful of families, which is why it has never been sold off to big resorts. Two main beaches face west and south, and you can cycle between them in twenty minutes.

Nothing here is dramatic. The pleasure is the scale: a beach for the morning, a plantation road for the afternoon, a beach bar for the sunset, and boats to smaller islands whenever you want them. It works particularly well as the middle stop between Chang and Kood.
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Boats leave from Laem Ngop pier near Trat, about an hour by speedboat and two by the slow ferry. Trat is four to five hours from Bangkok by bus, or under an hour by plane.
In high season, boats connect Koh Mak with Koh Chang and Koh Kood, which is what makes the three-island chain possible without going back to the mainland.
Most resorts arrange pier transfers, and the island's pickups meet the boats. Distances are so short that the ride is over before you have settled in.

Ao Kao has most of the resorts, the restaurants and the sunset. Ao Suan Yai is quieter, with a handful of places and the view across to Koh Chang. Neither is more than a short cycle from the other, so the choice matters less here than almost anywhere else.