If you want ancient sites and beach, the Yucatan Peninsula is a great place. You do not need to stay in Cancun. There are quieter, nicer places toward Tulum on the water. You can take tours from there to Mayan sites such as Chichen Itza, Tulum and Coba, or you can rent a vehicle and do it yourself. I bet our resident Mexico specialist, John Rule, can give you better details, but I have been there, and it was a good compromise between archeological sites and beach.

Lisa in Chicago