As Jo mentions, there are hundreds of islands to choose from. You can tailor to a wide variety of tastes as a result.
In our five month-long trips to Greece as a 50s-something couple, we have made it to 12 islands. Chronologically they are Corfu, Crete, Rhodes, Chios, Lefkada, Kythera, Paxos, Santorini, Amorgos, Naxos, Syros, and Evia.
The very best island in Greece, by the way, for you to visit is the one that you are on. Meaning, if there are any geographical forces at play, go with it; there are islands for everything in every island group.
I will share with you our most-recent Greece island trip which was a breeze for us to have done completely on-the-fly.
We got a great flight into Santorini and despite never planning on going there in the summer, we took out shot. It worked out well. We spent four nights on the caldera, hiked, visited historic sites, had great food, enjoyed the views, and took a long catamaran outing. This was in 2021 and there was a very small window allowing us to get into and out of Santorini in June without much difficulty because of Covid. We got lucky as the place was largely deserted. If not for Covid, though, we still would have jumped at the great flights and spent a couple nights in Akrotiri before moving on.
We were going to go to Folegandros next and then work our way up the Western Cyclades via Sifnos, Serifos, Kythnos, etc. On the day we wanted to travel, though, the ferry to Folegandros was not offered at an appealing time so we went to nearby Amorgos instead. That was a revelation and we stayed six nights.
From Amorgos we could see Naxos just over yonder so we took a quick ferry over there and stayed six nights. This was about July 1st so there were crowds on Naxos but it's a larger island and offered plenty for us to do without worrying about crowds.
After Naxos we had about 11 nights left to get back to Athens. I looked at the ferry schedule in Naxos port and decided on the roughly two-hour ferry to Syros. The best island in Greece is the one you are on so, without at doubt, for that week Syros was definitely the best island in Greece. We loved it, spending two nights in the city of Ermoupolis and three at the splendid beach village of Kini.
We had planned on hitting the next island in line geographically towards Athens but we were reading in the news at the time about a spike in Covid and potential coming difficulties in ferry transport. We had to fly from Athens eventually. I decided to just take the ferry from Syros to Athens and to get into a car directly and drive to only of only two islands in Greece that you can drive onto, Evia. As a result we were not at the mercy of any possible ferry disruptions prior to our departing flight. We spent our last six nights on Evia and had many wonderful and memorable experiences there while barely scratching the surface on the southern half of the island.
From there we drove to Mati and spent the last night of the trip 20 minutes from the airport. Easy car drop and out.