Suggestions for Thailand Road Trip?

I live in Jomtien Beach and have done many of the standard toursist destinations: KOh Samui, Phuket, Chiang Mai. I have the use of a friend's car for a few weeks and was thinking of hitting the road. Anyone have any suggestions? I don't want to venture more than a day's drive (one way) in any direction. I'll be traveling with my wife and 5 year old daughter. Thanks.