Peloponnese/Greece itinerary

Regarding the Peloponnese part I think this itinerary is quite good, and well balanced.
For once it's not a rush to see as many places as possible in as few days as possible while unpacking every day.

Just two comments:

• At the beginning, why not leave for Nafplio on day 2?

• I would not have added Naxos if it was to only spend 3 nights there (only 2 days). That’s a lot of travel logistics for such a short time in Naxos.

Instead I would have extended the Peloponnese until day 14 or 15 and ended the stay in Athens.

Without a doubt the best island in Greece is the one that you are on.

I see issues here.

First, you'll have to get into Athens, about a 45-minute drive from the airport. Then, you'll need to get back out to the airport to get the car that you are dropping off at the airport 10 days later.

Then, you have a nearly-four-hour drive from Olympia to ATH in order to fly to Naxos. Arriving early for your flight, that looks like about 7-1/2 hours of travel on Day 13. Days 14 and 15 will be fun. Day 16 is a long ferry to Piraeus and a taxi to a hotel. Then, the next morning the 45-60 minutes to take another taxi to the airport. Again.

If it's just a matter of going to an island, go to Kythera, Hydra, Spetses, Poros, Zakynthos, Aegina, Evia, or Kea. Those all fit in with your loop of the Peloponnese.

I would do this instead: Land, get immediately into your car, drive towards Nafplion (I have a place I stay midway but if you're fresh you can make it two hours). Do your Pelo things and whichever one island you want to tack on for three nights. Kythera would be my choice.

Then, drive into Athens, park the car in a lot for two days, enjoy the city, get in your car, drive to ATH, drop it off and waltz into the terminal.

If for some reason you have to go to Naxos, I would see if you can start there (fly directly there to begin your trip, ferry to Athens, etc.), or, actually, I don't see a way that Naxos makes any sense unless you are obliged somehow.

I would give Athens 3 nights to give you two full days in Athens and to recover from jet lag. I would do two nights in Napflion if you are only going to Corinth, Epidaurus and Mycenae. I agree that is a long drive from Olympia to the airport for returning the rental car and then catching the flight to Naxos. A previous poster suggested driving to Napflion immediately on arrival. Please don’t do that as you will be jet lagged and impaired mentally. That is dangerous.

Just a few thoughts, If you keep to your plan and will be in Naxos for 3 nights at the end of the trip you could opt to take a ferry from Naxos direct to Rafina. It is a port city and many of us choose to go there for our last night, Avra hotel is often recommended for that last night, Its on the waterfront with great views, its within walking distance from the port, is a short walk to the central square for your last night dining and it has free shuttle service to Athens airport.
We will be doing this exact route in June this year.

Many flights from North America tend to arrive in Athens just before or after noon. As previously suggested when you arrive Athens, if there is a convenient connecting flight to Naxos it may well be better to do Naxos first while you get over jet lag.. Getting over jet lag on in Naxos is always our best plan. You can do Athens at the end of the trip.

We drove from Nafplio to Monemvassia and got there in the late afternoon. (we stopped at Mystras which is on the road to Monemvassia. You may want to do Mystras on the day you transfer to Monemvassia). We spent one night in Monemvassia and headed into the Mani. Monemvassia is a memorable destination and one night just does not give you time to fully appreciate it. If you can find another night there you will not regret it.
When you visit Corinth do make sure you stop at Acrocorinth. It is a magnificent fortress with 360 degree views over the countryside.
For fun here are images of parts of your trip.
Nafplio https://www.flickr.com/photos/stanbr54/albums/72157632094108982
Corinth Area. https://www.flickr.com/photos/stanbr54/albums/72157711755443483
Peloponnese, Mani and Monemvassia. https://www.flickr.com/photos/stanbr54/albums/72157711323535897

If you really want to see Naxos, then you should. It's a wonderful island, and very different from Mykonos and Santorini. However, what you should do, as a few other posters have mentioned, is to fly to Naxos first. First fly to Athens, and then take the next available flight to Naxos. Make sure you give yourself enough time to get your luggage, go through customs, and board your next flight. Not only is an island a wonderful and relaxing place to get over jetlag, but you gain an extra night by doing this. With your current itinerary, you are spending time in Athens at the beginning of the trip and the end of your trip. By visiting Naxos first, you will have an extra night to add to Naxos, so 4 nights instead of 3 nights.

And I think 2 nights for Athens is fine since you have already been there.

Then fly from Naxos to Athens airport, where you will pick up your rental car and drive to Nafplion, about a 2-hour drive. I think 3 nights in Nafplio is a good amount of time to visit the 3 archaeological sites you want to see. And also have time to explore beautiful Nafplio. We followed a walking tour in my Michelin Green Guidebook, visited the Palamidi fortress, and spent some time at Arvanita Beach.