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You're coming at the right time of the year as it has dropped a few degrees, makes the evenings really pleasant.
I agree with Visit Thailand, you could check out Koh Chang - quite nice but very quiet also it's about a 4 to 5 hour drive from Bangkok.
If you haven't already you could take a trip 'up north' to Chiang Mai as it sounds like you've covered the beaches already.
Samui is always popular but if you were to head back perhaps try Choengmon beach which is not too far from Chaweng and is much quieter.
There is always Phuket as well.