Definatly go to Kanchanaburi and check out the tigers, they havn't eaten any tourists yet. Ayautthaya is definatly worth a visit too, hire a push bike and cycle round the various old temples best in the early morning or after 3 in the afternoon. Chaing Mai to Mae Hong Son via Pia is three hundred kms of stunning scenary. South of Hau Hin you'll find near deserted beaches at Khung Tanot.
As has been said take your time and enjoy "you'll come back"