This is a fabulous local comprehensive website gives you up-to-date information for everything you need to see and do on the island
The archeological zone of Chichen Itza, believed the famous Mayan pyramid was founded in 400 AC. EDIT in english--The trip begins with a home breakfast. The first stop is the archeological zone of Chichen Itza, where you could spend two hours. The trip continues in the Ik Kil cenote and then lunch at the colonial city of Valladolid, where you could visit the cathedral and the square. In the way back there will be time for a drink and a hotel afternoon nap. The local guides speak many languages and they are very funny.
Grab a bike and head to Mosquito island at sunset to see the flamingos. Just be sure to take bug spray, the islands name is no joke. The 15 minute bike ride is worth it
The prodigious Bird Island, Passion Island and Yalahau cenote.