add Delphi or Monemvasia to Nafplio and Idra/Hydra trip from Athens?

I drove to Nafplio on arrival (just under two hours), but I flew from the east coast, an easy, direct flight—if coming from west coast, I’d have needed a break before getting behind the wheel. Delphi is a slightly longer drive.
I don’t understand the two options as you have laid them out.

we land late and will overnight near ATH airport before picking up the rental car first thing the next day.

sorry I wasn't clear about my question initially.
our choice is to include Delphi or include Monemvasia (which literally means 1 way in /out and requires back tracking):

a. Athens -> Delphi (overnight in or near) -> Nafplio (3-4 nights) --> Ermioni (drop car) and ferry to Idra (2 nights)
or
b. Athens -> Nafplio (3-4 nights) -> Monemvasia (overnight) (with or without Mystras before or after) -> Ermioni (drop car) and ferry to Idra (2 nights)

RS rates Delphi higher on the must-see list than Monemvasia.

thanks

Driving in Greece, and especially in the Peloponnese, is always time-consuming (nothing like driving in California).

So, you'll spend more time in the car than actually sightseeing if you try to pack too much in. If you enjoy that, it's your choice.

Note, however, that in October, and even worse in November, archaeological sites close earlier; Epidaurus closes at 5:00 PM in November, Delphi and Mycenae closes at 3:30 PM.

You'll need to balance everything out so you don't end up standing in front of a closed site after spending hours in the car.

To my knowledge, the only car rental company that allows you to pick up a car in Athens and drop it off in Ermioni is Pop's Car:

https://popscar.gr/

You'll also need to check the boat or ferry schedules for the Ermioni-Hydra route. There is also a small local boat service that runs daily (weather permitting):

https://www.hydradirect.com/christos-ermioni-ferry

Once you've figured out these inevitable (and annoying) logistical details, you can start planning an itinerary that takes them into account.

Monemvasia is the one place in Greece I could have done without. That said, some people love it enough to spend a night, or even 2 there. Given your itinerary, it would also involve quite a bit of driving.

You’ll get more for your time and effort by not making the trip to Delphi and back. Focus on areas south of Athens. At Monemvasia, we got to hike up above the small town. There are lots of trails to go as high as you like, with plenty to explore. Mystras was also very worthwhile.

Our first trip to Greece was in 2018, where we also landed in Athens at night, then went straight to Hydra the next day. Afterwards we picked up our car (with Pop’s) in Ermioni, and stayed in Napflio, then Monemvasia, then headed farther south on the Peloponnese, to the Mani Peninsula. Continuing northwards, we made it to Kalamata, Olympia, Delphi, then Athens, where we turned in the car. With less time for a longer itinerary, Delphi will be out of your way from the rest. I’m guessing that even RS would agree that unless you’re planning an itinerary that goes farther west/southwest, Delphi wouldn’t override the other places you’re going to see.

I wonder about going to Hydra in late October/early November. I have not been there but I know a lot of the tourist infrastructure closes down for the season by then on other islands.

I have been to Delphi and Monemvasia and enjoyed them both. The only similarity they have is they are one night stays not convenient to Nafplio. Delphi is an important archeological site and museum. Monemvasia is a restored Byzantine town. It is totally given over to tourism and in my view best appreciated by spending the night there which allows you to explore beyond the gift shops. Delphi is more important historically but that doesn’t mean you would necessarily enjoy it more. We took our son and his wife with us to the peloponese this past May and I know they enjoyed monemvasia more than the archeological sites we visited. Similarly, on a different trip my twenty something kids liked Nemea much more than Mycenae.

I know this is really no help!! But I think in the end you need to think about what you honestly would enjoy more and go with it.

Now where we all enjoyed was Mystras which is closer to Nafplio and could even be a day trip. If you have the time, I would highly recommend it.