Last year we went with Pooh Eco-Trekking and had a fantastic 3 day hike and hill tribe stay. Many people have told me they had lackluster time trekking around Chiang Mai but ours was really good. Highly recommended.
The price is steeper than other treks - but we went with only the two of us. As well, the trek and the people we stayed with were incredible. We left no trash, brought lots of our own food in, and ate some rats and things the locals caught (tarantula egg sack is not fantastic like they say). We would totally go on another trek with them. So - not for vegetarians but there idea of low-impact includes seeing the locals sling-shot a bird from a tree for you to munch on for lunch - as they would.
59 Rajchapakinai Rd, Chiang Mai, 50200
Tel: (053)208-538 Mobile: (085)0414-971
Good to hear. Also here is Pooh's website http://www.pooh-ecotrekking.com/Pooh.htm. His office is just up the road from mine. I do the more mainstream treks and sometimes point folks to Pooh's place if they want something more specific.