Rainy season. Palenque should be nice, though it it will probably start raining ~4:00 ish and the road to Agua Azul may be dodgy.
Perhaps day trips from Cancun, or drive to and depart MID after visiting other archaeological sites, haciendas, etc.?
See the Climate Chart (
www.climate-charts.com) for Salina Cruz and Mérida below.