Peloponnese itinerary

2 nights in Monemvasia, day trip to Neapoli/Kythira?

As usual, I'm warning you that Monemvasia can be visited in a few hours. It's like a large open-air museum... No residents, just hotels, restaurants, souvenir shops, and tourist services. But if you'd like to stay there for 2 nights, that's up to you; it's nights you won't be spending in other places that might be more worthwhile.

You can't do a day trip to Kythira; it's impossible in one day given the Porfyrousa ferry schedule from Neapoli, and it wouldn't be worth it anyway. Kythira deserves at least 4r full days.

We have done a driving trip like what you are planning. It is a lot of driving. Bring your GPS for directions but also bring a paper map to be able to double check your route. We relied on the GPS only to discover that there are 7 places in Greece named Meteora.

We actually found Monemvasia to be a good destination. Its is a walled town and it makes you feel like you are stepping back in time when you walk through the gate and arrive at your period room for the overnight. Our room had a gun port hole in the wall.
Having said that you may not need the second night. If you need an extra day somewhere you could use that night elsewhere.
Since you have already been to Athens at the beginning of the trip you might want to look at a visit to the port town of Rafina. Its much more quiet than central Athens. Avra Hotel is a nice waterfront location at the ferry port and provides free shuttle service to and from the airport. It can be a relaxing way to spend you last night.

On your way to Nafplio when you stop at the canal you might want to take in Ancient Corinth and or Acrocorenth. Acrocorinth is a huge ancient fortress with 360 views over the countryside.

Here are some images of Monemvasia along with our drive around the Peloponnese.
https://www.flickr.com/photos/stanbr54/albums/72157711323535897/