Itinerary Advice? Athens/Santorini/Delphi/Peloponnese

Spent three weeks in Greece in 2012...loved every second. Here are my best attempts to answer your questions:
Didn't go to Hydra but 3 nights in Nafplio may allow it as there wasn't a huge amount near there. Corinth is not too far with some ruins and the canal. Don't remember how far to Mycenae, but go there and see site and museum.

We flew to Santorini vs taking the boat. There are also hi-speed boats. Be aware that not all boats go to all islands every day so check dates and travel times. We LOVED Santorini. But. we were there the year that protests in the country kept many visitors away so it wasn't crowded. Stayed on the side away from Oia.

Athens is a big, dirty city. I find I appreciate that first because by the end I am tired. Also go to the classical museum there. Most of the artifacts from Mycenae are here so having seen them first is a plus.

We hired a RS recommended tour guide in Delphi which I highly recommend.

Look at distances and travel time to help with decisions. ENJOY!!

Overall a good itinerary.

Santorini or Athens first? Your arrival in Santorini is late, so you may be very exhausted the next day. Perhaps that is good for the island and laying on a beach. But staying in Athens first might make that first day a bit easier.

Delphi for 1 night is pretty typical. Just check your timing to be sure you have plenty of time at the site (several hours including the museum). If timing allows, make a stop at the Hosios Lukas monastery on your way to or from Delphi.

Nafplio for 3 nights makes sense as you will likely arrive later that first night. There are lots of sites nearby to see, so you will fill those days easily. Many people say Nafplio is their favorite city in Greece.

Two full days in Athens will allow you to see the main sites and museums.

If visiting Santorini and Monemvasia, I'm not sure that the island of Hydra will offer much different. And you really won't have time.

Your plan is sound. Its too bad you don't have a few more days.
Definitely get to Santorini on your arrival day in Athens. Waking up jet lagged in Santorini is way better than doing it in Athens.

Renting a car from Athens airport is a good plan as you do not have to drive in Athens traffic.

On your way to Delphi think about stopping in Corinth. There are some great sites there. See Ancient Aptera, Ancient Corinth the mighty fortress at Acrocorinth. This stop will push back your arrival time in Delphi to late afternoon. Which is perfect. By late afternoon the tour bus crowds begin to depart. Find a hotel and visit the museum. Perhaps visit the lower site. Have a nice quiet evening and next morning get to the gated site first thing. You will have the site virtually all alone for a couple of hours. By noon the place will be crowded so get back to your car and drive to Nafplio. Perhaps this would be a good time to do Ancient Nemia as you drive right by the site on the way back towards Nafplio.

You are going to love Monemvassia. Perhaps on the way you might consider a stop at Mystras. It was the last Byzantine city and is fairly intact particularly the Cathedrals.

Make Santorini your island experience. Not much need to do Hydra if you have already done an island. If its a must for you then it can be done from Nafplio. I have never been tempted to do that visit thought.

Here is your plan in pictures.

Santorini http://www.flickr.com/photos/stanbr54/sets/72157632154800782/

Nafplio and Peloponnese http://www.flickr.com/photos/stanbr54/sets/72157632094108982/
Corinth area sights https://www.flickr.com/photos/stanbr54/albums/72157711755443483

Athens http://www.flickr.com/photos/stanbr54/sets/72157632121475515/

Thank you for the responses! I really appreciate it.

When is your trip?

The coastal towns of the Peloponnese are much like the island towns. All you need in Santorini is 2 days, however it may be very crowded with tourists (more so than anywhere else in Greece), and not such a good place to chill.

I went to Hydra on the RS Greece tour. I think it's more of a novelty (no cars) than a great destination.

Like your plan but may be tempted to drop Delphi since it's in the opposite direction. Would save you a lot of driving time and you could add a night between Athens and Monemvasia. Also, after returning from Santorini you could drive all the way to Monemvasia and then work your way back. We visited Athens and Santorini in 2016 (3 nights each - about right) and are planning to revisit Athens and well as travel to Hydra and Nafplio and the Costa Navarino area in May of this year. Although we've never visited Delphi, my feeling based on research and photos is that you will get a similar (but different) experience by visiting the Acropolis and Agora areas in Athens. Also, there are ruins between Athens and Nafplio.