A must go place is Merida, in the state of Yucatan. You will love the cheap old hotels, I can recommend good ones. Try to get one near downtown, walking distance from Plaza Grande. I would say take an air conditioned bus from Cancun, about $30 or less if you book a bit ahead. 5 hours trip. You could go from Oaxaca too but it would be a longer trip. But you could split it up to make it more tolerable. There is an airport in Merida also.