Ko Samet
 

If you're travelling to Thailand and want a truly beautiful tropical island away from neon lights and fire eaters, head to Ko Samet just to the west of Bangkok. You won't regret it.

Koh Samet is southeast of Bangkok, off the coast of Rayong Province. It's a good couple of hours in the car or on a bus from Bangkok, then about 40 minutes by ferry to reach the island.

When I visited several years ago, I found it pretty but very rustic.

I can assure you that Samet is not nearly as "rustic" as it was a few years ago.:) The nice thing about Samet is that you can choose to stay at beaches where there's a lot of day and night time (but not TOO late) activity or stay at other beaches that are very quiet and secluded, all within a couple of hours Bangkok. Hire a speedboat in Ban Phe and you'll be delivered directly to the beach of your choice in about 15 minutes. On the weekends Samet is a very popular destination for 20- and 30-something Thai.

Some prior discussions of Samet:
Ko Samet Tips?
Best 2 night trip from BKK?
Koh Samet: Still Paradise?

Ko Samet has a nice laid back feel to it. Most lodgings are still rustic but a few resort-type places have opened. We stayed at Paradee resort for a few days last year. They have some nice pool suites and although a little quirky and service not quite perfect, it was very nice.
The busier beaches with many reasonably priced bungalows seemed to attract lots of european families and at weekends there were, as you have said, many Thai visitors.
We enjoyed many meals there outside the resort and sunset drinks on a beach as the tide came in and covered our feet with warm water...its a lovely island.