Travel route. Where to go?

Personally, I prefer the culturally inspired holidays. So I would recommend going where there is something interesting to see and schedule your itinerary around those places of interest. A route going up north from Bangkok to Chiangmai is rather 'classical' and a good start.

Going up north from Bangkok, interesting places to visit are : Bangkok, Ayuthaya, Lopburi, Sukhothai, Sri Satchanalai, Chiang Mai. I have underlined the 'must see' places. To the west of Bangkok, Kanchanaburi may be of interest, especially to those who have still war memories.

While the province may be of interest for those who have time and want to relax in a pleasant green environment, it does not warrant a great detour, if you are on a tight schedule. Close to Chiang Mai, Lamphun is worth a half-day trip. Chiang Rai should be rather optional, there is not much to be seen at the so called Golden Triangle. So again, depending on your time available, you may skip this area. Phitsanulok, in between Bangkok and Chiang Mai, can be a good base to visit Sukhothai and Sri Satchanalai. Sri Satchanalai was the sister city of Sukhothai during the Sukhothai kingdom, and has some interesting ruins, though not as extensive as the ones in Sukhothai itself. All the cities above are connected by bus routes. Day and nighttime trains go between Bangkok and Chiang Mai, go and stop in Phitsanulok, but do not go through Sukhothai.