Chiang Mai? Spent five weeks there and it has a good vibe, mix of traveler & locals, nature & culture.
Discussion
I want ocean, my bad. Should have included that
Koh Samui in that case. The lesser popular island of the two; rent a scooter and go to the hidden beaches.
Still would recommend Chiang Mai tho (even for a detox), take the loop to Pai and go to the canyon, geyser, waterfalls, elephants and temples.
But for ocean life Koh Samui was great too.
Sorry, Ko Pha Ngan actually. Samui is the busy one in the south of it, Ko Phangang is the tranquillo one of the two.