I visited CM last Christmas and had a great time. Temperature and humidity is much more manageable, the City is very walkable and the sights are excellent.
Street food is varied, inexpensive and delicious.
If you are into natural pursuits (hiking, paddling, animals) then there is a lot to do. No beaches.
I also visited the Tiger reserve, which was quite an experience You get to pet and hang out with GIANT TIGERS (people still think the pictures are fake!). Also visited a no-ride elephant camp (do some research on ethical elephant attractions).
Having been to BKK several times, and having previously visited Phuket and Koh Samui I still had a wonderful short stay in CM.