Are you really that big into ruins? If not, seeing Chichen Itza should be sufficient, which would leave you time to explore Merida (great city; at one time, the wealthiest in the world), visit Progreso ($2 bus ride from Merida; retro beach town a couple miles from Chicxulub, where the meteor that killed off the dinosaurs hit Earth), etc.
Depending on your arrival time in Cancun, you might be able to catch a bus right from the airport to Merida, which eliminates the need for a rental car for that long, boring trip. (The highway is inland and there's absolutely nothing to see along the way.) The CUN-Merida ADO bus drops people off right next to the main hotels in Merida. (There's a Hyatt, Holiday Inn, InterContinental, and Fiesta Americana all at the same intersection.)
As of a couple months ago, there was a red-eye bus from Merida to the Cancun airport that leaves Merida around 11:00 PM and gets to Cancun around 4:30 AM, which is perfect for early-AM flights.