I did 3 months backpacking SE Asia last year, got to most of the major backpacker spots and some of the quieter, less well-known places as well. For a honeymoon I assume you won'y be wanting to stay in cheapo hostels like I did so I'd recommend:
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Railay is one of the most amazing beach spots in the world. Great climbing as well. There are also some nice upscale resorts to stay at. The only negatives are that the beach does get somewhat crowed but not too bad, and the dining/nightlife options are somewhat limited.
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Koh Samui is a nice option if you're looking for a more westernized and developed option. Plenty of nice mid-range and upscale resorts. Nice beaches, many dining options and good nightlife. Unfortunately the island is fairly crowded with tourists, although I'm sure you can find more secluded areas as it is quite large. You also have the option of taking a boat across to the Full Moon Party if you are so inclined - without having to stay in the backpacker party madness of Ko Phangan.
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Ko Tao is
the place to go for diving, it is known as having the best diving in Thailand and there are probably 100 dive shops on the island. A very chilled out and relaxing spot but there is a little bit of nightlife. The beaches are nice, but not spectacular.
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Koh Lipe is my suggestion if you are looking for a place to forget about the world for a couple weeks. It's more rustic than the other places I've mentioned, there isn't even an ATM there, but there are loads of private beachfront huts that you can have to yourself. The beaches are amazing, and diving is decent.
Places to avoid:
- Koh Phi Phi is a great place to party with Scandinavians, but the beach is full of seaweed and sea lice, and I don't think there are very many places to stay that would be suitable for a honeymoon.
- Similarily, Ko Phangan is backpacker party central (especially Haad Rin Beach, the site of Full Moon). There are some other nice beaches further up the island, but they are not accessible by road, and you are at the mercy of crooked boat operators who charge exorbitant rates.
- I'm on the fence about Phuket. I only spent a day there and it felt like Thai Cancun to me. It's a huge place though and there are some
very nice luxury resorts away from the crowds that would be worth a shot.
- Pattaya is full of fat, old sex tourists and Russians. It's quite possibly the sleaziest place in the world.