Mother/tween daughter first time to Greece Itinerary

1st of all, don’t drive in Athens. There is a cheap, clean metro system; and you can take taxis, where it doesn’t go. Just be careful about pickpockets.
I suggest you add a night to Delphi and another to Napflio. Skip Naxos. I advise that just because traveling between destinations eats up lots of time.

Pick up and drop off your car rental at the Athens airport. It's then freeway (toll road) around Athens going north.

While Milos is indeed magnificent and Naxos is my favorite of the dozen or so islands I've been to, I believe that the very best island in Greece is the one that you are on. Each has their own unique charm and beauty - in abundance.

Therefore, I really don't feel it necessary for you to depart Nafplio after Day 8 to get to Milos. My goodness, that's one hell of a travel day. It's a 2-hour drive from Nafplio to Piraeus and then what do you do with your car? For whatever reason, there is very little rental car infrastructure near the port. Getting from car return at airport to ferry port is minimally another hour. Assuming there's a convenient ferry departure time, it's going to be a 4-5 hour ferry to Milos.

The first 8 days here are very good. Personally, there are a great many individual locations in and around all those places you'll see on Days 1-8 to make all that effort just to get to a particular island. That list is available upon request. If, however, you absolutely must be on an island, keep it easy and go to one of the ones that is near to where you'll be.

The beaches on Lefkada are some of the most beautiful in the world - and you can drive there by bridge! Corfu is as-famous and gorgeous as Milos, and it's near to where you will be. Zakynthos is sublime. Kefalonia, ... the prettiest. On the southern end of where you'll be I would recommend Poros, Spetses, Hydra or, a real curveball, Aegina, which would be a good choice if you want to lose all the non-Greek tourists.

I have been in your shoes. 8 years ago, while planning the first of what has become five one-month trips to Greece. You will never be disappointed going to some place closer and box-checking is not required. Hope this is helpful.

Frankly, with just 14 nights, I would choose one or the other, a loop tour of the mainland north of the Corinth Canal, or a loop around the Peloponnese Peninsula. You could easily spend a month or more for either choice. If you go north, I would go Delphi (including Hosios Loukas, Arachova, Galaxidi) 2 nights, Meteora 2 nights, Parga 2 nights, Lefkada 3 nights, Nafpaktos 1 night to break up drive, and the rest in Athens. If you prefer remote mountain villages to beaches at any point, you could sacrifice Parga or Lefkada time in favor of Epirus mountain towns. This route could also easily add Nafplio on the way to Athens. You need to be in Athens on your final night, so make it your final 2, 3 or 4 nights.

If you choose south, I would go Nafplio (2 or 3), Monemvassia (via Mystras) (2 or 3), Island of Kythera (3 nights), Gythio (2), Olympia (1 or 2, depending on your expected arrival / dep times), rest in Athens.

And, don't worry about driving in Athens too much. Any hotel I've dealt with would gladly handle my car for me if I warned them in advance that I was coming directly to them and nervous about locating parking on my own. Or, note your driving route to the hotel and locate a parking lot en route and nearby on a major artery. That will keep you from the difficult side streets.