August and first trip to Greece

One week on the mainland. No islands?

I would cut one night in Athens and add it to Napflio.

You could reduce time in Athens and consider how important Delphi is to you. ATH to Nafplio is an easy drive and the region is quite pleasant for exploring.
You can't change the weather I am afraid, but there are beaches galore.

Thank you for the suggestions!

Experienced travellers to Greece often recommend getting away from Athens as soon as you arrive. Right now you are spending 3 nights in Athens then your last night there as well. Why not rent the car from the airport on day of arrival. Head to Nafplio and spend two or three nights in Nafplio plus Delphi. Return to Athens for the last 3 nights. Logistics count on a short trip and this way you free up a bonus day outside of Athens.

I would get into a car at the airport and drive to Nafplio (if it's late at night I would stay at hotel just before Corinth Canal), stopping at Corinth Canal. Spend your last two nights in Athens and then fly home. Divide the other five nights between Nafplio, Nafpaktos (or Corinth, Patras, Galaxidi), and Delphi. Personally, I would go 3 Nafplio and one each Nafpaktos and Delphi before driving back to Athens.
From Nafplio you can easily visit Mycenae, Argos, Epidavros and many more differing kinds of places. On the way to Nafpaktos you can stop at Corinth castle or ancient town, or both, or take the train up to Kalavryta. Visit the castle at Nafpaktos and its great view of the bridge over the Gulf of Corinth, Visit Delphi museum in the morning (sooner the better, tour buses are coming from Athens), shoot over to Arachova, maybe, for lunch (or Hosios Loukas monastery if you're still game), return to Delphi ancient site for 1800 entry (will be as close to deserted as possible, buses long gone). Finally, two-hour drive into Athens (ditch car in 10-euro/night kind of lot), hit the sites hard for two days, then get car and drive to airport for departure.

These are all awesome tips. Thank you!