go to the island of Redang, its really small, no cars or planes, the only way to get there is by boat. theres only 4 hotels on the island and its suuper chill.
what i highly recommend is the john grey sea canoe trip that takes you to the islands of the Phang Nga bay, there are parts that are breathtaking.
also if you can arrange a jungle trek in Chiang Mai, its great, you walk through the jungle with a guide, and live with the tribe people at night in huts, you walk through old opium fields and rice paddies, its a great experience.
and if your into downhill mountain biking, there is are world famous downhill and XC tracks that are alot of fun if you want to change things up a bit.
i was thailand and malaysia for 3 months.
if you pass through malaysia, check out Taman Negara national park, and make sure to check into the taman negara resort that is across the river from the town, great cultural experience there.