Going on a Trek with a guide is the best way to see the hilltribes.
Experienced guides will take you through the maze of tracks in the hills to villages where there is a good chance of seeing something of the traditional way of life. Good guides will also be able to explain the tribal spirit world and associated rituals.
Trekking is a well-established part of the tourism industry and has necessarily affected the hilltribes and their economy. After more than thirty years, no area in northern Thailand can therefore be "new". Treks often include an elephant ride and a bamboo rafting trip. Some tours may also include the opportunity to smoke opium, a practice which makes money for the hilltribes, but which is definitely illegal.
You do not have to follow the beaten path if you decide to go on a trek. Some trekking companies offer alternatives. You can organize your own group and then approach one of the companies and work out what you want to do.
Expect to pay at least 600 Baht per head per day for a reasonable tour if the group is between 6 and 10 people, and more if there are less people.
When trekking, travel light. Keep at least one clean change of clothes dry for wearing in the evening and night. Warm clothing and a sleeping bag are essential in the cold season. A rain poncho (sold to protect motorcyclists from rain and widely available for around 70-90 Baht.) serves both as a raincoat and ground sheet.
Basic medical supplies to sterilize and cover cuts, settle upset stomachs, and sterilize water may be useful.
Use mosquito repellent and clothing to keep mosquitoes away in the evening and early morning. Malaria is more of a threat near the Burmese border than close to Chiang Mai. Do not rely on either a mosquito net (you may not be able to put it up) or an anti-malaria drug.
Good hotels should have a reliable safe-keeping service for personal valuables, but there can be no guarantees. In the past, a few travelers have found that credit cards left for "safe-keeping" have been used in their absence.
Whether to take yours with you and keep it hidden away, or leave it along with other "valuables", is your choice. When visiting remote places, keep valuable possessions hidden from sight.
See also: [Trekking Companies] [Visiting Hilltribe Villages] [Malaria Prevention]