Cozumel is only worthwhile as a base for scuba divers. Playa del Carmen is much nicer than either Cozumel or Isla Mujeres. If you want a different overnight excursion that makes it easy and convenient to get to Chichen Itza early, take the ADO bus directly from Playa to Vallodolid the afternoon before and experiece an authentic colonial Mexican town. The "real Mexico." There's a cenote very close to centro and also the most recently uncovered Mayan ruin is not far (sorry can't remember the name right now). It will be more natural and far less crowded than Chichen Itza, if you are up for two ruins.