I spent 5 days in Chiang Mai, part of a 6 week trip in Southeast Asia.. This was a great place, especially due to my fantastic hotel, Pak Chiang Mai (check it out on this website)
One of the best experiences was my trip to the elephant rescue preserve called Heaven. This place tries to rescue as many elephants as it can from the horrible abuse that is done to them in this country and I assume, everywhere. It is at least an hour away up in the beautiful, mountainous countryside. You get in the river and help bathe the elephants and also feed them. You will have a whole new understanding of these magnificent animals after a trip here.
I also enjoyed a day at an organic farm cooking school. You learn how to make 7 different dishes, including using a stone mortar and pestle to grind the many ingredients of curry paste. The food tasted just terrific and they give you a book at the end of the day with all the recipes.
A great first day experience was hiring Paul Collins to take us around the city including temples, the cultural museum, having lunch, and visiting surrounding craft villages where he gets no cut if you buy anything. He is the perfect guide because he was born and raised in Thailand but his parents are American so his English is completely American. I did not really understand how difficult it is to understand the English spoken by so many Southeast Asians until I traveled here. Also, he considers himself Thai yet he is completely in tune with American culture too which gives him a different perspective than most guides. We asked him incessant questions which he seemed very happy to answer. I found him on this forum and want to add to the accolades.
We also rode the shared red truck up the mountain to the temple that overlooks the city, definitely worthwhile. We road on to the winter palace, so, so, and on to the very commercialized hmong village which was a beautiiful ride but not an authentic experience if that's what you're looking for.