Plot it all on a map and see where the outlier is. I suspect it's Delphi, which is too bad, but that's what I'd cut due to geography.
I was going to say Olympia because when given the choice between Delphi and there I always recommend Delphi. The sites are very similar but Delphi comes with a 10-out-of-10 view.
On the assumption that you are departing from the Athens airport, I would get your car upon arrival, drive to Nafplio, then to Monemvasia (stop at Mystras if you have time), to Kardamyli, Olympia.
To be honest, getting back to Athens and then to Hydra and back is logistically difficult. You could park your car for two nights at Piraeus and take the ferry from there. Or, you could choose a different island (Kythera or Poros work very well for your route).
Then, last two nights Athens, drive car to airport, drop off, walk into terminal.
If you do it the other way, counter-clockwise as it were, you'll have an unpleasant airport-to-ferry connection, followed by having to get a car in Piraeus (far fewer options than at the airport) and, presumably, returning it to the airport at possible added cost.