What is the best Thai island for a married couple in their early thirties to visit, looking for beautiful beaches which aren't over run with tourists, somewhere which hasn't been commercialized yet, also bearing in mind that we are going in November which is rainy season for alot of thailand?
You'll want to visit the Andaman Sea (Western) side that time of year. I would recommend Ko Lanta. Avoid Phuket, Koh Phi Phi, Railay Beach, and even Koh Lipe , as they are now completely overdeveloped and overrun with tourists. That said, November is the start of the busy season, so if you come a little early it might not be as bad.
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I love Koh Kood and the Koh Kai islands! Been hearing a lot of good stuff about Koh Bulon Lae too, though I've neve been there. Hopefully soon!
Hi - I am also in my thirties and I really enjoyed Koh Chang. And I've heard really good things about Koh Mak, which is smaller, more remote, and right next door. I was going to check it out last month, but as a solo traveler I thought it would be too quiet. For a couple, I bet it would be perfect. Just one ferry between the two islands so you can easily do both. High season starts in Mid November so depending on when you go, you may catch the start of it - but it may not be too bad yet.