If you are in Corinth you have to see Ancient Corinth and the Temple of Apollo. It's an extremely significant area that defines how the land and cultures were shaped thousands of years ago. Everything is well preserved and accessible. Dress for heat as everything, much like the rest of Greece, is sandy and wide open.
This is definitely a place to stop on your way from Athens out to the Peloponnese. It is just as interesting as any of the ruins in Athens and no people out here. As you learn quickly in Greece, there is a Temple of Apollo at many sites.