I'm sort of the opposite - I love Chaing Mai and would spend as much time there as possible! Krabi (or more so Ao Nang) is great for relaxing, but there's not a great deal to "do" there (except day trips out to the islands).

In Chiang Mai I did a brilliant cooking class with Asia Scenic (http://www.asiascenic.com/) - Gayray, the lady who runs it, is a scream and so patient. When I go back, I'll actually go to the same cooking school so I can try some other dishes! Since getting back, my BF is loving it - we eat thai food at least every second night.