Chang Mai is my favourite part of Thailand, it's what I imagine Bangkok was like 50 years ago.  It has a great night market, lots of day trips in the area, and great food.  A real must is to do a one day cookery course, most of the hostels will organise them for you.  The course generally involves a trip to the market to buy your food for the days cooking followed by lots of cooking and eating, a great day out and I guarantee that when you return home you will use what you have learned to impress your family and friends.