How is Chiang Mai like if you've ever been there, and can you give suggestions on the best tourism spots?

Yes to all your questions. I've stayed there 6months. The weather in Dec is good, only a slight chance of rain and you'll only need a sweater if you're on a Honda at night - then a coat is a good idea. Stick to tuk tuks and taxis and you'll be OK with light clothing even at night.