I find staying on the river in Bangkok relaxing, such as at the Marriott Riverside. Then you can decide whether to chill at the hotel, take the boat across and plunge into town, or take an excursion from the hotel such as to the Bridge over the River Kwai and Tiger Temple. The hotel has its own boat for dinner cruises, and the staff can arrange for your own long tail boat ride of the klongs.
I've been to Chiang Mai recently. Unfortunately, I was there when air pollution was extremely high. There is an interesting range of things to do and see. Starwood has just opened a new well located property.
It really comes down to how much flying you want to do, and whether a river respite appeals in Bangkok.