Short 1-/2-night trip from Bangkok (by plane)

I will be in Thailand for a few days mid-January. I've been to Bangkok couple of times and also some of the common destinations in Thailand (Chiang Mai, Phuket, Krabi, Phi Phi, Ko Tao, Ko Samui, ...) so I am looking for something a little bit different and "out of the way".

Are there any recommendations for a destination worthwhile for a short 1-/2-night escape from Bangkok? I have no problem at all with flying and am not on that tight of a budget.

What I am looking for is preferably a relaxed place, a swimming beach would be great. I don't want to spend too much in transit after landing, so not four minibus rides only to then be on a ferry for two hours.
An alternative to a beach would be one of the secondary cities in Thailand, on google flights I can see many cities with good connections from BKK/DMK all around Thailand - Any recommendations?

Thank you!