Chiang Mai old city was founded 1296 around and it was built as a walled city surrounded by a moat, which you can still see the remains of today.
Most of the walls have collapsed, or the bricks have been used for other constructions in Chiang Mai, but the corner towers still stand almost intact.
Each side of the moat is about 1.6 kilometers, so if you walk all around, it is a walk of 6.4 kilometers. It doesn’t sound of much, but in the daytime, it can be very hot, and as you very seldom have cool breeze in Chiang Mai, but instead have all the traffic, it can a very exhausting walk, especially if you are not used to the heat.
I can really recommend that you take a walk around at least some of the Chiang Mai old city moat. I think the most interesting part is the east side where you also have Thapae gate in the middle between the southeast and northeast corner. That is also the side there has the most street life.
A nighttime walk is maybe the perfect choice, it is cooler and there is light on the old walls.