None of the cafes or restaurants along the Corinth Canal out of town or along the waterfront in town deserve to be singled out; most should be avoided. Similarly, the restaurants near the ancient site cater to tour groups and tend to have high prices and mediocre food. However, here are a couple suggestions if you find yourself hungry in Old Corinth (and can't wait until you get to Nafplion).
Locals speak well of the Splendid restaurant across from the site; a lunch or dinner of simple stews, chops, and salads costs around 15€. The Ancient Corinth and Taverna Dionysos on the main square offers much the same food at similar prices.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.