Is it better to stay 6 nights in CM and leave for a day visit and come back, or is it better to stay "only" 3 or 4 nights and prepare a trek in North or elsewhere? Thanks
While you're there you should check out Fah Lanna Spa. Wonderful massages!
3 days in Chiang Mai is a good choice. You can stay longer if you love a quiet vibe here. It's certainly not a place if you love going to beaches. You can view my friend's itinerary for example. She gets all the needed information for a trip to Chiang Mai.
Was there for 5 nights and it was not enough! Wish I could have stayed for 2 weeks. If you fly into Bangkok, get out of there immediately and go to Chiang Mai:) Have fun!
I suggest spending your time in Chiang Mai straight through, but you should do lots of day trips based out of Chiang Mai (hiking/walking, elephant preserve, cooking school, etc.). Then head out on your hike move on to the next part of your trip not having to go back. CM is fantastic - have fun!
Chiang Rai is also a good place to spend a few days or a week & you used to be able to do a 3 day bamboo raft trip down the Mae Kok from Mae Chan to Chaing Rai= best trip ever.