Greece in October Athens/Nafplion/Monemvasia best way to travel

Athens airport to Nafplio you can rent three cars and drive to Nafplio in 2 hours. Perhaps you can rent a van some of the rental agencies do have larger vans for groups.
Having the cars allows you flexibility. For instance on the way to Nafplio you can stop at Corinth to see the canal plus ancient Corinth and Ancient Nemia. A sister to Olympia and still has its stadium.

There is lots of parking in Nafplio. You won't need the cars while touring the old town area however they wil come in handy if you want to visit Palamidi fortress. Its 999 steps up but it does have a sneaky back road. I have come to appreciate drive up Fortresses.
Nafplio is perfectly situated for day trips to the nearby antiquities.

On the way to Monemvassia you will drive right by Mystras the last Byzantine city. You should plan to visit on the way to or back from Monemvassia. No the car won't fit through the city gate and once inside the cernter street is wide enough for a donkey and a table and chairs. Actually if you are sitting at the table you have to duck otherewise you will get brushed by the donkey. Very charming.

With the three cars the people returning early to Athens can take one so no return issues.

Too bad you don't have a few more days you are going to be rushed in Nafplio and Monemvassia and surrounding area.
Nafplio and Peloponnese http://www.flickr.com/photos/stanbr54/sets/72157632094108982/

Stanbr thank you so much for your thoughtful and detailed response. If we were to add a day, would you suggest Nafplio or Monemvassia? Also, do you have recommendations for what to do with the car while in Monemvassia. I’m thinking we will stay in the old part of the city. Thanks again!

Add to Nafplio. It makes a good base to see sites in the such as Myceanae, Epidavros and Mystras. Plus it is a mid size city with lots of restaurants. Two nights in Monemvasia should be adequate.

I also agree with have 2-3 cars. Public transportation in Greece is not that great or convenient. And with the group returning early, you'll need flexibility. If people pack light, you might fit 4 in a car. But if all four have large bags, there is no way they will fit in a European car.

Stanbr, I was thinking Nafplio would be the better choice too, so thanks for helping me make up my mind. And thanks for the link to the photos. My husband is also a photographer, so he will enjoy viewing them as well.

Best, Susan