So in my opinion the route that you should advise your friends from Bangkok:
Since BKK, they can go to Ayutthaya and back in one day, the former imperial city so full of old temples, even in BKK I like the flower market in Chinatown, very early in the morning, a place really genuine.
Obviously, there is Wat Pho (buddha prone) and the royal palace when one goes there for the first time, fewer tourists, I also like the market for amulets, which is not far from the Royal Palace but that one only really appreciates after having spent a few days already in thailand to assess the intensity of religion in daily thai life. O
f course the evening, Patpong can not be missed nor the resto "banyan tree" in Patpong precisely. Or the Somboon side Silom, which proposes a crab curry delirious (but REALLY spicy).
Once you have done a little BKK, we can either go in the north, or go through Sukhothai, another imperial city and even monuments.
Otherwise we can go to Chiang Rai (city not very interesting but a practical departure for the North) and find a car and a driver to go to Mae Salong, the Chinese city of the former Kuomintang who exchanged their opium crops against that of tea.
We must spend a night (Mae Salong Villa hotel in the best location, or Khum Nai Pol which is more intimate ) for the atmosphere of Chinese village that can be drawn, the women who deliver their crop of the day at sunset…
some images here:
http://www.trekearth.com/gallery/Asia/Thai...photo153508.htm http://www.trekearth.com/gallery/Asia/Thai...photo156054.htm.
I really discourages "golden triangle", which is a major tourist scam, nothing to see but a lot of shops. We must also be careful oftravel agents and guides: it ranges from very good to the unbearable, in Chiang Mai I strongly advised Fortune travel north star, "i use them each time, always very nice and much cheaper that agencies,
Chiang Rai I had a very bad experience with an a....e who spoke french but did not want to show it, which is located just next to the hotel Wiang Inn. In general, you have to avoid the bus tours. to see the tribes truly authentic requires a driver with a car or stay in ecolodges.
Moving from Chiang Mai, they can also stop at Tha Ton or fork to Angkang which is a very nice mountainous corner (it's called the Swiss Thailand) with Palang tribe villages. Then they go down again on Chiang Mai: it's best to sleep in the ancient city to fill the evening walking around to visit the temples around: http://www.trekearth.com/gallery/Asia/Thai...photo161378.htm
From chiang May , we still need to take a driver to go discover the tribes Lisu, Lahu, Karen, or Akha Palang to Chiang Dao
www.trekearth.com/gallery/Asia/Thailand/photo97719.htm and also see performances snakes in northern Chiang Mai (we tell ourselves that we are a real boy again, but it's up to scream of fear! a try imperative).
Then, it is imperative to do the Chiang Mai / Mae Hong Son loop. This is where the most beautiful scenery of Thailand and the tribes arethe most interesting.
Doablein 3 days, we can stay much longer, according to its own pace. From Chiang Mai goes to Mae Hong Son where there is a magnificent temple on the heights http://www.trekearth.com/gallery/Asia/Thai...photo161844.htm
and tribes Padaung woman "giraffe" http://www.trekearth.com/gallery/Asia/Thai...photo170243.htm
then continuing to Pai, landscapes become truly magical http://www.trekearth.com/workshops/167248/photo31399.htm
and tribes most authentic http://www.trekearth.com/gallery/photo167248.htm.
Pai is a paradise for baba cool and smoke, on the door of the guesthouse, there are marked please do not smoke joints in bed… what is pathetic is the horde of tourists it attracts and that from 11 am in the morning you see that there is no longer anyone to cross the street without being nearly killed. Nevertheless, there are some interesting eco lodges in the area that can share the life of the villagers (I think the next time I go is what I would do) I do not remember the names but it in the region between Chiang Mai and Pai.
A friend who knows much better than I Thailand (been three times a year over the past five, six years to make the picture) said that the area of Mae Sot is even more authetique but I do not know .. Same for the Issan very rural region of Thailand ...
But I hope nevertheless that it will give a first indication of things to do in North Thailand.
But I have been locked up by the Thai context, and I realize that it is INDIPENSABLE to spend time in Luang Prabang in Laos, impressive city of zénitude with thousands of temples and a little more monks. Faith is palpable, especially in the morning when they charity in the streets.