1-3 are small charming cities with hardly any Farang. You will see lots of mountains and small farms and basically how Thai people live. These areas will be chilly in December so be prepared.
4 is the second largest city in the country but still much smaller than Bangkok metropolis.
You can find all sorts of things to do there but again not hordes of visitors.
Will you be renting a car or going by bus?
The roads are small and winding through the mountains so take your time.
I recently purchased two adjacent properties, one with a house, this way. Chiang Mai area. Any Thai adult and any foreigner adult can make this contract. Thai statue does not specify married couples .