@Tom ... lots of things to do in Thailand, but I have never been there. I know a lot of people who have and a couple of my best friends lived there for over a year, so I have had innumerable conversations about the place.
I don't know if you're traveling solo or not, but I can't recommend Thailand enough if you are, especially if you're not 100% comfortable doing so. There are hostels, guesthouses, and nowadays Airbnb basically everywhere, enough that you don't have to speak the language of the locals in order to get by. It's popular enough that they're used to tourists throughout the country, even in the isolated areas, but rugged enough that it's still authentic. Sounds cliche, but according to everyone, it's all true.
Geographically, I would suggest staying in the north if you're not a beach guy (I'm a fat bum who doesn't take his shirt off, so...) and like the misty mountain scene and the indigenous culture, the south if you like the sun and the girls giving you massages, and the east near Cambodia if you want to get into the middle of buttfuck nowhere and risk dying of malaria, which is by far the more adventurous thing, then find a bus with no air conditioning and head out into the hills. I don't really think you can go wrong, but do lots and lots of research on what exactly you want.
I know it's on my short bucket list.