Hope this mix of info might help on some level. We visited the Peloponnese and Athens at Eastertime last year, and are going to Crete in late September/early October this fall.
Because we got to the Peloponnese by boat from Athens and Hydra Island, we didn’t pass near Corinth, so no advice there. After Hydra, once we were on the peninsula with our rental car, we drove clockwise, taking about 2 weeks to make it to Archaea Olympia, so it would be a long out-and-back trip in a day from Nafplio. BUT, there’s an amazing, less well-known site much closer to Nafplio, called Nemea. It also hosted athletic games, and has its own stadium site (in better shape in some ways than Olympia’s), where you can line up on the starting blocks and do your own 200 yard dash, maybe with no one else around. The stadium has a unique entrance tunnel you can walk down, and see the scratched-in-stone graffiti from athletes 2,000 years ago, whose Greek “I won!” Is still visible. There’s also a smaller but wonderful museum, plus a temple site with some re-erected columns, and sheltered foundations of other buildings. You could conceivably stop by Nemea on your way to Napflio, or do it on a trip from Nafplio. A real, virtually undiscovered gem!
Are you planning on hiking up the hillside above the shops and restaurants/cafes inside Monemvasia? That will get you some distance away from many other people, but mid-September will likely be much less crowded than mid-July or mid-August. You’ll get a sense of the crowds based on how far from the entrance gate you have to park, maybe even on the mainland, not along the causeway. A worthwhile visit, if you can make it. We were able to spend a night there, and trek up the hillside just after breakfast.
The day we were headed for the Mani peninsula (far southwest, too far for a day trip from Napflio), we passed near Mystras. We didn’t wind up making a stop there, largely based on understanding that there’s really no physical remnant of ancient Sparta there, kind of like there’s no Bastille in Paris anymore, although the place it once sat is there, but you’d never know it from being there.
Enjoy your time in Epidavros and Mycenae! Last year, the big domed tomb just down the road from Mycenae was full of hornets, so we didn’t stick around in it long.
