I just got back from an amazing trip in Thailand.
We stayed mostly in Koh Lanta (Klong Beach) which was lovely, quiet, with amazing food, and has easy access to snorkel spots/surrounding islands like Koh Ngai, Koh Rok, Koh Phi Phi.
We stayed at the Lanta Sand Resort & Spa (which was a bit 'family friendly' but lovely all the same). There is loads of cheaper/more expensive accommodation, from camping, huts, to Pimalai Resort. Agoda.com had pretty good recommendations.
If you are in Koh Lanta, be sure to check out Thai Cat, Funky Fish and Suza Hut for dinner (& order one of the sizzle plates). Same Same But Different is meant to be yummy too, though we never actually made it there!
Koh Phi Phi is tonnes of fun, and amazingly beautiful, though I would only recommend it if you are staying on the quiet side of the Island, or at the Phi Phi Cabana (which has the most amazing pool right on the beach)! It can get a little crazy/dirty/smelly, but it is tonnes of fun!
Further down south than Krabi, toward the Trang Province, is less touristy and the beaches are amazing.
My friends were recently in Railay (Krabi). Whilst it is beautiful, they said it was busy and flooded with tourists, as was Koh Samui. Koh Tao is meant to be amazing for diving (though I haven't actually been there yet!).
If you are 'stuck' in Phuket for a night (as we were) I would HIGHLY recommend staying at Burasari. It's a 'sanctuary' off the main drag at Patong and we were thoroughly impressed!
We met a nice tour operator (sadly at the end of our trip) who offered us really good rates for our next Thailand trip. His name is Jay and I'm sure he'd be happy to provide you with more advice! (Jay - Yavavee Jarong ).
Let me know if you have any other questions about Thailand or need any other contacts!
Hope that helps :)
Ps. We haven't uploaded our photos from this trip yet, but you can check out videos from our snorkeling escapades in Koh Rok, Koh Ngai etc here... http://vimeo.com/armyofdolls/videos and photos from our Thailand holiday in May here http://www.flickr.com/photos/armyofdolls/collections/72157621288242257/ about 10 years ago
Koh Phi Phi is probably your best bet. Having been to the islands on both coasts, it has the best combination of white sand, sprawling beaches and secluded spots. You can join in the hubbub and nightlife of the little town, but can also remove yourself from the crowd as well - it was by far my favorite island in Thailand. Make sure you take the hike up to the top of the mountain for sunset. about 10 years ago
I like Star Huts on Koh Phangnang - take a ferry from Koh Samui, go past Ko Tao and get off at the next island! If you ring in advance, they will arrange for a jeepnie to come and collect you from the ferry.
In August 2009 I stayed at Tub Tim Resort on Koh Samet which was a nice place, although slightly large. The resort is at the end of the beach - about a 10 minute transfer from the ferry. It has a huge variety of rooms from delux suites, nice one beds with balconies and AC to small rooms, no AC, no electricity. The food at the restaurant was great and you could walk as far down the beach to other places as you fancied. The sea was lovely for swimming in and the staff at the resort were good.
Its about a two hour drive from Bangkok and then a 20 minute speedboat or ferry transfer - I would go with the speedboat as it takes you directly to the resort of your choice. about 10 years ago
If you want a secluded beach experience in Thailand I'd recommend Koh Mak in the north of the Gulf of Thailand - just near the Cambodian border. Most tourists head south to the stock standard beach resorts so the northern islands are a welcome reprieve from drunken hordes of European backpackers.
Having said that the southern beaches on Koh Lanta are quiet and secluded too. about 10 years ago