All of these are excellent suggestions. It really depends on what you are looking for apart from BKK.
We loved Chiang Mia with the elephant sanctuaries, the orchid nursery, the temple etc. Lots to do but very chilled.
If you want beach, Hua Hin (or just North) is fantastic and easily accessible from BKK. Suggest a private taxi. If you stay more central, there are a few clubs, pubs, restaurants etc but generally not a party place so not wild.
You could take a speed boat to Koh Samui from there and hope you have decent weather. On our trip, we barely beat the ferry!
KS is great but you want to stay on the Chaweng side. However, between our first and last visit there a few years ago, it has become more built up but not as bad as Phuket.
Koh Chang is a trek and does have a few more mosquitoes than elsewhere but very quiet. You could become bored of love it. You decide.
Personally, I would skip Pattaya as I found it too much but again, you decide on how you like your holidays.
But BKK a must. I love the buzz and the feel and always start a trip there for 3 days and then relax elsewhere afterwards.
We discovered a lovely village called Huay Yang about a 6 hour drive from BKK down south. Depending on when you go in your time frame, you will get a great deal as it picks up quite a lot towards the end of that time as the Swedes move in. Well worth the travel.
Finally, close to BKK if the River Kwai. Well worth the visit especially if you like whistling the tune. You can even stay for the night on raft boats which was incredible especially if you want to include a river raft experience.
