Epidaurus was once a major city, most well-known for its healing qualities. It served as a sanctuary, with hundreds of spas and structures devoted to the gods Apollo, Asklepios, and Hygeia. The most important spa, Asklepieion, was declared a UNESCO World Heritage site. You can still see the shrine to its namesake god, as well as the remains of rooms for patients.
Epidaurus' main draw, however, is its impressive theater, with seating for 15,000 people. It was built in the 4th century B.C.E. It's still used for events even today.