Just come back from Chiang Mai a fortnight ago!
We went out with Mountain Biking Chiang Mai, second time we've been out with them. No need to book, there are tour shops all over the city, they all have leaflets and will book you a ride for the next day.
Haggle over the price a bit as we got it cheaper than advertised in the leaflets, then give what you save as a tip to the fantastic local guides!!
Bikes are OK, don't expect disc brakes or clipless pedals! Take your own armour and back pack. They won't let you ride even xc without armour and their backpacks STINK!!
Good mix of trails, wasn't too hot as the rides start high in the mountains, but don't forget the insect repellent.
Have a fab time..........v jealous, I want to still be there!!!