Need help planning a trip to Greece

I think you have too much on your list, so I'd first tackle the low-hanging fruit. Most itineraries here include Athens, Santorini, Mykonos, and Crete, so if you peruse some other itineraries, you will get a sense of the travel logistics. That would pretty much fill two weeks for me. If you share how many days you want to spend in those places, it will better enable others to advise you on how to include some of the rest. Some will work just fine for a traveler who likes to move at a quicker pace, but some are likely outliers that you need to save for a return trip.
So, get a handle on the easy parts first, then you should be able to priortize the remainder. Good luck!

When you first arrive, get a domestic flight to the first island on your list. Save all your time on the mainland for the end of your visit so you aren't caught in an unforeseeable situation where you can't get back to Athens for your outbound flight. Strikes, mechanical breakdowns, cancellations and even high winds can happen at any time without notice. Use the Ferryhopper website to help you plan your island hopping. For any destination that interests you check the corresponding travel forum on the TripAdvisor website. Click on "Things to do" at the top of that forum's page so you can get a good idea of what's available to you in terms of activities. Try using the filter system at www.booking.com to search for the sort of accommodations you require at the price you want to pay. Be sure to go to your local AAA office to pick up an International Driving Permit, no test or membership required. Bring a couple of passport photos to save a few dollars.

I agree, that's a lot for 2.5 weeks, but a little more in-depth research will let you know if your proposed itinerary is overly-ambitious or not.

Hi Jeff, 2 ½ weeks and you’ll spend a few days in Athens? Assume you’ll be back and see what you don’t have time for on this trip.
Meteora and Vikos are on the mainland in northern Greece. I’m not sure when in Sep you’ll be there, but Meteora is a place you want to visit in the fall when it’s most beautiful due to the fall foliage; something you don’t experience in CA. This may be a good time to include Mykonos, Milos, Santorini and Crete.
Voidokilia Beach and Monemvasia are located in the Peloponnese Peninsula (southern GR) and are quite doable from Athens. I would drive from Athens to Monemvasia (4h) and then to Voidokilia Beach (3h 15m) and then to the Kyllini Ferry Terminal (3h) to get to Zakynthos. You can transport your car on the ferry and it’ll take 1h 15m: www.levanteferries.com.
Unfortunately, to get to Kefalonia from Zakynthos you’ll need to go back to the Kyllini terminal and change ferries to get to Poros on Kefalonia (1h 30m). You’ll need to return to Kyllini and drive to Lefkada (3h).
From Kyllini you can drive to Vikos (2h 45m) and then back to Athens (5h 30m). If you drive to Athens from Kyllini it'll take 3h 30m. Vikos and Meteora are 3h apart if you decide to combine it with Meteora.

Looks like a great trip! Avoid driving in central Athens if you can avoid it - and I'd do Athens last. After arriving rent a car from the airport and head out on the driving portion of the trip first. Then double back and drop the car at the airport before flying to the islands - or in Piraeus before taking the ferry (Piraeus can be tough driving but it's not too big). There are trains from both the airport and Piraeus to central Athens so when you get back from the islands you don't have to worry about a car. Then you can enjoy all that Athens has to offer for the remainder of your trip without worrying about logistics other than the train out to the airport.

Greece is harder to hop around than you think. If you want to see islands in different island groups (Cyclades vs Ionic, for example) you often have to return to Athens. Consider sticking to one island group as it will make life much easier. Take a look at ferry schedules to get a sense of what I mean.

We were in Greece in 2018 and went to four places (3, 4, 4, and 3 nights). I certainly would not try more than 6 places in 2.5 weeks. And even then, it depends on where you want to go. Crete, for example, is huge and many people spend 1-2 weeks just there.