There are loads of great places to see and stay at. I guess it all depends on what you like and what sort of accomadation you want. If you have never done anything like that in Thailand then it will be a great adventure. When I did it I had no plans at all. Just hired the car for a week and followed a map. I would also recommend a sat nav with some poi's. That way you will always be able to find a hotel (even in the middle of nowhere).