The main reason people head to Chang Mai is because it is the adventure capital of Thailand.
The best action/adventure trip available in my opinion is the elephant riding and bamboo rafting jungle trek. These excursions can be done in just one day or over a period of 5 it depends how long you want to camp in the jungle for. I did the one day trek and it was superb. You start with a 40 minute walk through the jungle, tiring yes but so beautiful up to a small village tribe.
As well as taking in the stunning views across the jungle you are able to see how the villagers go about there lives. Following the trek you are taken by mini bus to where the elephants live. This is your chance to interact with the elephants and ride them around the jungle. After an amazing time with the elephants you are taken to a waterfall and then on to bamboo rafting. The whole experience as a whole is fantastic and beautiful, a real culture experience.
You can book this excursion in the majority of Chang Mai tourist agents or hotels/guest houses.