Chiang Mai is nice, but as one of the previous posters said, it is chock full of rude, entitled wannabe hippies (you know, the rich kids trying to be cool, scruffy backpackers). Having said that, it’s a nice place, and light years better than BKK.
I know your trip is already planned, but if you come back, or want to make a diversion for a few days, I highly recommend Chiang Rai. Not so touristy, real Thailand. Take a day trip up to Mae Sai which is right on the Myanmar (Burma) border. You can also see the “real” Golden Triangle, Wat Rong Kuhn (the White Temple) - just an all around amazing place.