Been to Thailand many times.

Places recommended checking out are:

ayutthaya - the old capital of Thailand, lots of historical stuff like temples etc to get abit of history while on your journey. Take a trip on a rice barge etc.

Kanchanaburi - see the effects of WWII (and if you are an aussie like me you can check out hellfire pass and also the war cemetary to get an understanding of what aussie POW's went through). One of my favourite towns in Thailand.

Trekking Chaing Mai - check out the hill tribe people etc.

I use the following people on my trips and find them really good.

ayutthaya-boat
good-times-travel Kanchanaburi
nutty-adventures trekking, bicycling and other stuff


Hope someone finds this useful.