Thailand's far north-west Andaman coast, an hour from the Myanmar border. Two bays, concrete footpaths instead of roads, and hornbills in the trees.
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Koh Phayam sits off Ranong in Thailand's far north-west, close enough to Myanmar to see it on a clear day. It has no cars at all — the island is crossed by narrow concrete paths just wide enough for a motorbike, and motorbike taxis are the transport.

That single fact keeps the island the way people come for it. Between the two main bays are cashew plantations and rubber, and hornbills are common enough that you hear them before you see them. There is no town, no bank strip and very little that is open late.
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Ao Yai on the west coast is the main beach and the reason most people come: a long stretch of sand facing straight into the sunset, with bungalows and a scattering of bars along it. In the shoulder months it picks up enough swell for beginners to surf.
This is where the island's small social life happens — a handful of beach bars, mostly low-key, mostly closing early. It has the widest choice of places to stay and eat, and it is the sensible first choice unless you specifically want quiet.

Ao Khao Kwang, usually called Buffalo Bay, is the north-western bay: shallower, more sheltered, with rock outcrops and a calmer sea. It is the better choice with small children or if you want to swim rather than watch waves — though at low tide the water goes a long way out.
The eastern side of the island is quieter again, with a working pier, mangroves and a couple of small bays rather than a resort strip. Wherever you stay, the island is small enough to cross by motorbike in twenty minutes.

Ao Yai for the long sand, the sunsets and the widest choice. Buffalo Bay if you want calmer, shallower water and even less going on.
Buffalo Bay is, with sheltered shallow water and no traffic at all. Just plan around the tide, which goes a long way out.
None. The island is crossed by concrete paths wide enough only for motorbikes, and motorbike taxis do the transporting.
By boat from Ranong. The slow boat takes roughly two hours and the speedboat far less. Ranong has an airport with flights from Bangkok.
More Koh Phayam and Andaman coast guides, plus the rest of Thailand's islands.