Lots of great options in thailand and you live like a king for 50-60 bucks a day.
You want islands? Head to the south. Koh Tao (turtle island) is where I learned to scuba dive, and eventually took the wife back. Take a ferry to Ko Samui which is great. Good flight options from ko samui to get back to bangkok or over the mainland to phuket.
Like somebody said earlier, Chang Mai up north is a laid back feel. Don't pass on Bangkok though, aside from all the dirty stuff you can get into if its your thing, there are really cool markets (floating market, MBK) you can go see, and you can get a really nice hotel room for cheap.
Best thing for us was the 6 dollar foot massage for an hr... while drinking a huge beer that cost $1.50.
Safe travels... PM for more info.
CC