Thailand's northern lobe is one of the best-developed parts of the country; traveling through it is easy. Roughly circular, its form suggests drawing a circle to cover it efficiently. Moreover, the climate in the north is the best in Thailand, thus it is the perfect place for acclimatizing to the heat and humidity.
Phitsanoluk is the ideal place for starting a big loop around Thailand's north due its position as the biggest crossroad in the country, sitting at the main connection of the North, with Isaan and Bangkok. There are buses from here practically to all of the north; buses from everywhere to everywhere stop at its bus terminal at all hours.
The best is beginning the loop northwards; despite all the ways being equally good in their transportation's facilities, there is an important difference in the direction in the stretch from Nan to Chiang Rai. This is the most scenic route in the whole path, a tortured and narrow road advancing up the steep mountains towards Chiang Rai; in the opposite direction the bus speeds down and thus there are less opportunities for enjoying the way.
Northwards, Phrae is the first town reached. Air-con buses from Phitsanulok to Phare cost 116B and leave at 7:00, 9:10, 11:30 and 15:00. Placed by the Yom River, this secondary town was never important enough to enjoy a development process, thus it offers the visitor views of a classical Thai town, though all it is left of the old walls is an elevated path.
A regular bus from Phrae to Nan travels the 117km way for just 47B. Nan was the last principality to join the Thai Kingdom, as recently as 1931; Muang Nan was an independent kingdom since the mid 14th century, in parallel with Luang Prabang and Lan Xang in Laos.
The town hosts an interesting temple, Wat Phra That Chang Kham, which has a stupa held by sculpted elephants; the only one similar to it in Thailand is in Sukhothai, but there the temple is inactive.
Two ways lead from Nan to Chiang Rai; the newer passes through Phayao and has several buses during the day. However, the famous and scenic one goes through roads 1080, 1148 and then 1021. Only one bus per day uses this road; it leaves at 09:00, arrives at 15:00 and costs 130B. Staying overnight at the town of Chiang Kham, crossed around 13:00, is possible.
Chiang Rai was once the capital of the Lanna Kingdom and became a Thai province only in 1910. Nowadays it is a modern and comfortable city, the second in size in the north after Chiang Mai and the perfect place for replenishing the backpack or sitting idle for a couple of days. Being the gate to the Golden Triangle, it is the ideal place for starting and finishing that itinerary.
Doi Maesalong is an interesting detour from the loop; to get there take the bus from Chiang Rai to Ban Pasang (from 06:00 to 18:00 every 15 minutes) and then a truck from the village northern junction to Doi Maesalong. The village is one of the counted Kuomintang (Chinese Nationalist Party) settlements in Thailand and is famous for its tea and views.
The next main stop is Chiang Mai; this stretch of the way takes you from the eastern part of the loop to the western one and there are several ways of doing it. An interesting way of reaching Chiang Mai from Chiang Rai is through Fang, a village along the old way connecting to Chiang Mai, buses to there leave at 8:00, 10:00 and 14:00 and cost 49B (137km, three hours).
On the new route connecting Chiang Rai with Chiang Mai, the distance is 180km and the way longs anything between three to five hours, depending on the class of the bus. Regular buses cost 77B and leave between 6:30am and 5:30pm. Air-conditioned buses cost 139B, and leave between 8:45am and 4:00pm. On Fridays and Sundays there are additional buses at 4:30pm and on Fridays at 7:00pm. VIP buses cost 216B and leave between 8:00am and 5:30pm. The old route is significantly longer, 338km, longs six hours and regular buses leave between 6:00am and 4:30pm every 20 minutes, and cost 83B.
There is an additional option to travel through Lampang, 225km, 5 hours and 81B. However, it is more sensible to save time visiting Lampang from Chiang Mai.
Chiang Mai is the second most important city in Thailand and has many points of interest for tourists; see my specific entries for them. One of them is the Mae Hong Son loop.
If travelling through Mae Hong Son or skipping it, Tak would be eventually reached. Most buses leaving for Bangkok from Chiang Mai pass through it, but there is a direct, regular bus from Chiang Mai's Akaed terminal at noon (platform 15, 137B, three hours). West from Tak is Mae Sot, from where renewing the Thai visa by crossing to the Burmese town of Myawaddy is possible.
East from Tak is Old Sukhothai, which is reached with any bus travelling eastwards from the terminal for 28B. It is better to explore the ruins of the old capital from Old Sukhothai; the new town is away from the ruins and has no special characteristics on itself. Trucks do the twelve kilometres way between the two for ten baht.
Closing the loop from New Sukhotai to Phitsanulok costs 23B; there are frequent buses between the two during the day.
Phitsanulok is the comfortable way to end your journey, though, if in a hurry, returning from Sukhothai to Tak is possible; from there are many buses to Bangkok. Except for the express buses connecting Chiang Mai to Bangkok, all of them stop here; buying a ticket on the spot is possible. Usually they have available seats and are the fastest way of reaching the capital.
The whole loop, including the Chiang Rai and Mae Hong Son detours can be done in less than a week without stops; However, a good plan should allow something around one month for pleasantly enjoying this wonderful part of Thailand.