We are leaving for Thailand tomorrow morning. It will be our 3rd time going. We are skipping Bangkok and going straight to Chiang Mai this time because once you have done Bangkok a couple times, you feel like you are done with it. Koh San Road and MBK quickly lose their appeal. The water taxis are a cool way to get around. Definitely check it out if you’ve never been, but I wouldn’t recommend more than a week.
We love Chiang Mai and you will definitely love it there. Big enough that you can find everything you want, but not crazy like BKK. Lots of different food options and a very walkable/scooterable city. One of the best produce markets in the world if you really like fruit (durian anyone? ) and fresh produce. Probably our favorite place in the world (so far!).
We also spend time on Koh Phangan island in the south. Really beautiful there. You can go for the parties on Haad Rin (more touristy/EDM crowd) or the Black/Half Moon parties (more hippy feel and local) on the south side. You can also go for the pristine and quiet seclusion. We love The Sanctuary on Haad Tien on the south east tip. It is like no other place we have been. In the north there is also yoga and scuba.
We have also been to Railay on the other side of the mainland a little bit south. It has really nice beaches and rock climbing. Not really a place to live though as it’s pretty small and there is not much available, but really cool to visit. There is a very cool hike called “Lagoon” that takes you to the top where you can see all of Railay. Can be treacherous at points depending on the mud/rain situation.
We have done Phi Phi as well, but it’s pretty touristy and feels chaotic.
We have also heard good things about Chiang Rai and it’s temples just south of Chiang Mai, but we haven’t been (yet!)
Hope this helps. You made a great choice coming to Thailand