Ko Lipe might suit you. It's got some nightlife (getting busier every year in fact but it's not Phuket or Ko Samui yet). Good snorkelling and side trips to uninhabited islands like Ko Adang and Ko Rawi where there is some of the best coral in Thailand. Check Ko Lipe and other Andaman islands out on
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Island Hopping Thailand: hotels, resorts, travel information
Quieter islands (with less nightlife) are those around Trang.... Ko Ngai, Ko Kraden, Ko Muk, Ko Bulon Le, Ko Sukkorn, Ko Libong or you could try Ko Phayam or small Ko Chang near Ranong. Big Ko Chang is quite developed now but there are smaller satellite islands near it...Ko Maak, Ko Kut, Ko Wai.