Which beach to visit?

Hi!

My wife and I are planning a 4 week trip to SE Asia. Looks like the most logical and common thing is to fly in and out of Bangkok (we're coming from NY). At the end of our trip before we head back to Bangkok for our return flight we wanted to spend 3 nights at one of Thailand's beaches. Looking for something rather quiet away from too many crowds but still beautiful and would like to stay at a place that is a bit upscale (maybe $150-200 per night) but not something like a Hilton resort. My mind is on something cute immersed in beautiful nature ha.

Any ideas for both beaches and accommodation would be greatly appreciated! (Also, my wife and I are both vegan...just in case that jogs anyone's memory of some amazing place they visited!)