We're finally avoiding Koh Samui so we'll start in Bangkok, then will head north (CM, CR, MHS...) and then to Cambodia, we decided these because going south (as we will move by bus/train) would cost a relatively high amount of time. So, our plan include: Bangkok, Ayutthaya, Kanchanaburi, Lampang, Chiang Mai, Chiang Rai, Mae Hong Song, ¿Ubon? and then in Cambodia: Siem Reap, Phnom Penh and Sihanoukville.
Thanks to everybody who answered, and keep travelling.