If you are visiting Nafplion or Mycenae you are likely to pass close to Argos. The area has been inhabited since Neolithic times and Argos was a rich and powerful city in the Archaic era 1000 BC alongside Mycenae. Today Argos is a modern town with some small archaeological sites remaining, but most tourists head straight on for the major sites.
The Theatre at the western edge of the town was built in 4th century BC and renovated by the Romans, once seating 20,000 though half of the seating has been lost. It makes a good short visit as its by the main road out of town and you can park right outside (when it’s 40C in the shade these considerations become important). You’ll probably have the place to yourselves too, apart from the guy in the hut who I’ve never seen come out yet. Whitman