Been a few laps around SE Asia.
Thailand is great, I love it, will go again, it's easy but a bit less adventurous than others
Vietnam: Interesting, cheap, does get a bit samey - they tend to be quite conformist so everything's much the same in one region (food/shops/etc) so you have to travel for a difference. Stayed a month or so here which was a highlight
www.google.com.au/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=vietnam%20phu%20quocCambodia: interesting, lots of history, slightly edgier like when you pull into Pnomh Phen (sp?) they have lots of guards around bus and give lots of theft warnings etc. Stayed in worst room ever, a $5/night on stilts over the lake (now filled in) with a rat highway thru it in the middle of the night.
My pick: Laos.
We stayed somewhere about $35/night US for double massive hut with bathroom & balcony w river view. Included 3 hot meals /day for 2 of us, french cooks, homemade bread & local coffee for breakfast, etc. Free bikes, fishing rods, kayaks. One of the only rivers in SE asia totally clean and inviting to dive in. Lots of local trips organised etc. Could have happily stayed weeks.
Can't put my finger on the exact one but this kinda thing is representative of what you can find..
http://ecotourismlaos.com/eco_accommodation/index.html