Potentially stop at Ayutthaya temples, only an hour north of bangkok. There is a local train from Bangkok’s main Hualamphong station to Ayutthaya roughly every hour or so with basic but clean 3rd class seats, taking a leisurely 1 hour 45 minutes for the 71 km (44 miles). Costs around 50p: http://www.seat61.com/Thailand.htm#Bangkok_to_Ayutthaya.
Second stop could be Sukhothai, which is half way between Bangkok and the Northern territories. No train station at Sukhothai, but you can take a comfortable train from Bangkok to Phitsanulok on the Bangkok-Chiang Mai main line, which is about 59 km from Sukhothai by bus. Buses leave Phitsanulok for Sukhothai frequently between 07:00 and 19:00, fare around 40 baht or so, journey time 1 hour. On arrival by train at Phitsanulok, simply ask one of the tuk-tuk taxis waiting outside the station to take you to the nearby bus station. This train/bus combo avoids a nightmare 7 hours on a bus from Bangkok. The 08:30 or 11:50 DRC express railcars from Bangkok would be a good choice to reach Phitsanulok, see here for train times. Once capital of the Sukhothai Kingdom, the UNESCO-designated ruins are 12 km outside Sukhothai town, easily reached by local transport: http://www.seat61.com/Thailand.htm#Bangkok_-_Sukhothai
Sukhothai hostels: http://www.hostelworld.com/findabed.php/ChosenCity.Sukhothai/ChosenCountry.Thailand
Things to do in Sukhothai:
Next stop potentially Chiang Mai:
Looks to take 5-6 hours on a bus, but an easy popular route: http://www.hostelworld.com/findabed.php/ChosenCity.Sukhothai/ChosenCountry.Thailand
Things to do in Chiang Mai:
Nice Elephant place: Patara Elephant Farm.
Wat Prabat Si Roy
Bua Thong falls
Mae Kampong Village
Wat Pha Lat
Takes 3/4 hourish from Chiang Mai, Starting to become a bit more touristy, but near the Mae Hong Son Province which sounds remote.
Things to do in Pai:
Tha Pai Hot Springs
Sai Ngam (secret) hot springs
Pai Canyon: AWESOME place for sunset
Waterfalls: Thailand is jam packed full of the things, so if you haven’t od’ed on watefalls already, check out the two best in the area – Pam Bok waterfall and Mae Ya waterfall.
Unexplored areas in the North, including Chiang Rai: http://www.lonelyplanet.com/travel-tips-and-articles/70219
Mae Hong Son province – least populous region
Soppong – great town with caving and hiking