I've only been to Goa in India and Phuket/PhiPhi/Krabi/Phang Nga in Thailand from the countries you intend to visit, and those are the tourist centers of both countries which in my understanding many backpackers tend to avoid. Still if you end up in either, there's a lot to see and do. I especially know about the areas in Thailand since I've been to Phuket for at least five times already. One thing I recommend the most from seeing the list of countries you're going to: Scuba-dive! If you haven't done the course yet, do it in for example Thailand and you won't regret it. I've done diving in a lot of places around the area and would especially recommend the area around PhiPhi islands. Managed to see sharks, turtles, sepias, pufferfish, lionfish, etc there. I've heard that the tsunami hasn't really hurt the reefs either...
Another activity that offers some stunning views, is the kayak/canoe(/what ever you call those things) rides in Phang Nga area. There'll be a guy paddling the kayak to magnificent lagoons and at least one long cave full of bats. The whole area in itself is worth seeing from the sea, with it's amazing high tropical islands (familiar from an old Bond movie) sticking out from the sea. Very "touristy", but that shouldn't put you off doing it if you get the chance...
You can also just browse around the brochuredesks in the bigger tourist cities (like Patong in Phuket) for a ton of cool activities to choose from yourself.
I'd also like to say that I'm still incredibly jealous and that's exactly the kind of trip I'm intending to do sometime in the (hopefully near) future, when I get the chance.
