The Cave of Maronia is the area's most atmospheric natural sight, linked in local tradition to the mythic world of Odysseus, Maron, and the wine that overcame the Cyclops. The cave usually needs a guide, so check locally before you build the day around it.
Its appeal is not just underground scenery. The cave ties the ancient city to Mount Ismaros and to the stories that made Maroneian wine famous in Greek literature.