Leave Chiang Mai very early in the morning.
Your first stop will be wat Pa Dara Phiram Phra Aram Luang, a long name for this temple complex very beautiful and interesting.
If you're an onsen lover like I am, head to Mon Jam for the views, and then to Onsen@Moncham, it's a wonderful place where you really feel like Japan. After your relaxing time, head to four seasons for lunch.
Here if you have more time, I'd suggest to stay the night either in onsen@moncham which offers rooms or in 4 seasons if you have the budget for a private villa.
After lunch, head to Wat Ban Den, it's a bit of a detour for Pai, but it's worth it, that temple complex is huge and beautiful.
Then direction Pai and the numerous turns on the road. On the way you can stop to Mok Fa waterfall which is very nice and a short hike from the parking lot.
Finally you arrive in Pai. If you have time for sunset, head to Pai Canyon, but if no time, go the next day in the morning after sunrise, you'll have zero crowd.
Spend the evening in Pai walking street and night market.