Ko Kood is a beautiful beach destination but it does take the better part of a day to get there, and another better part of a day to return - ground transport to Trat, longish ferry ride together which can be challenging to time/sync.

Sukhothai, IME, during Loy Krathong is not that great. Sure there are tons of "events" but the whole experience can be lost what with all the electrical cables strewn about, crass commercialization and a gazillion vendors. I've returned at other times and found the experience much better. If you do go to Sukhothai make sure to explore the dozens of temple ruins nearby (to the west, outside the old city walls), and Wat Phra Phai Luang and Wat Si Chum.

If you are going to Chiang Mai during Loy Krathong/Ye Peng - and it would be better there than in Bangkok - you would be well advised to book transport/accommodations sooner rather than later.