Sounds like you've got plenty of ideas already. What sorts of things do you like to do? I mean, what is it you love about Thailand?

Surely you've seen enough of Bangkok? If you must spend time there (and I don't blame you) save it for the end of your trip so you can do all your shopping then. Spend a few more days up north instead. After Chiang Mai, explore Chiang Rai. Head up to the Golden Triangle and you can do a quick trip to Laos and maybe Myanmar (if the border is open).

I really enjoyed my visit to Koh Samui, so I would have to recommend it, but I prefer the beaches of Phuket. If you're any sort of a party animal, consider Pattaya. I got dragged there a few years ago and found it to be not nearly as bad as I had imagined (and been told). The beach is generally wretched though.