Itinerary suggestion 1.5 weeks Greece family of 4

Any of those are possible but not all in the time you have. Are you spending three nights in Athens? Not clear from your numbering but I would.

We have made two trips to Greece and between the two trips have seen everything on your list of possibilities.

You could go to Delphi and Nafplio in that time. Delphi is in the mountains and the views are amazing. The museum there is one of my favorites because it is small and everything is from the site. It has amazing pictures of them unearthing statues. It also is inexpensive to stay in and a nice contrast to Santorini which is pricey. I would stay 1 night there.

We went to Nafplio with a family group of seven and really loved it. We stayed 4 nights. We were there in early June and it was hot. We went sightseeing in the morning and spent the afternoons at different beaches. Your kids could easily wander around the town which I am sure they would enjoy. The castle there was several of our groups favorite activity. To avoid the heat, we left our apartment at 7:30 am and climbed the 999 steps to the top. Another favorite activity was Nemea which was one of the sites on rotation with Olympia for the ancient games. My kids lined up and ran a race. Some of the ruins are reconstructed which makes it easier to imagine. And the museum there was interesting and air conditioned!! We appreciated everything more because it was comfortable.

So one option would be 3 nights Santorini, one night Delphi, 4 nights Nafplio, 3 nights Athens. I would fly back to Athens from Santorini and rent a car at the airport. Drive to Delphi from the airport. After 1 night, drive to Nafplio for 4 nights. Then return the car to the airport and go into Athens.

Totally agree with Beth! The Nafplio/Argolid area is ideal for history, beaches, hiking, castles ...morre more more -- AND its sunsets "eclipse" those of Santorini. Adding to list of highlights is, en route, just off the main highway to Nafplio, is ANCIENT NEMEA... the "rrunner-up to Olympia" as ancient-games site. It's been authenticallyl excavated... inclluding stunning stadium where your kids can race from starting-blocks 2400+ years old (!). Here's a great onlline map- https://tinyurl.com/zekuft9 - showing bypass highway from airport to Pelopoinnese... NEMEA is 5 mi or so to right of road, jsut after Corinth Canal. BIG BONUS -- Greek Enthusaists have established the Revived Nemean Games every 4 years -- https://nemeangames.org/en/home-english/... and June 28-30 2024, thousands will be participating, with traditional clothes, ceremenies etc Even if u are not there for the event, you can experience the pre- or post-games excitement & activiities.

NAfplio & area ideal for active, curious youngsters... this superb NONcommercial website is packed with info, click every link! http://www.visitnafplio.com/ I also agree on strategy of AM/sightsee, aft beaches... Local Tolo Beach can be very crowdy, but go just a mile or so beyond, to Asine/Kastraki... a small sandy curve & a pebbly cove, deilightful swims, NO Crowds, one cute taverna (where u can change in restroom), nice for snacks or earlly dinner. In Nafplio, be sure to find the Elevator IN the clilff (ask your hotel) going up to the Akronafplia ramparts... and do the sea--level walk around the point, to the rocky Arvanita Town beach (NOTE: going to Greece, all of u bring cheapie $6 "water shoes" not flip-flops, so u can enjoy any Greek beaches). As for famous Palamidi "rock" of 999 steps, a cool strategy is to DRIVE up back way, take pics from on high... then one parent & kids can go DOWN the steps, while the other parent drives to lovely cafe at foot of the mount to have coffee & await them (that is my chosen role). Nafplio at night is especially magical, the marble-paved town square is lively with families... kids playing soccer under the moon... So festive!

I guess the "Santorini catamaran" is a few hours catamaran cruise. In this case you will only have one full day left, which means that even with a rental car you will not really be able to explore the island. At most, you can go for example to the south of the island to visit the Akrotiri site and some villages/beaches.
I warn you that Santorini's beaches are not great and also that it is not an ideal island for kids.

Santorini→Piraeus by ferry takes between 5 and 8 hours.
Even if you take the first ferry you will not be in Piraeus before 5 p.m. / 6 p.m.

Taking an early flight to Athens and renting a car at the airport would be smarter and would allow you to leave straight away for Nafplio or Delphi. (approximately 2 to 3 hours drive from the airport).
Delphi or Nafplio, you will be there at the end of day 4.

Meteora is almost 5 hours drive from the airport. Not only would it not be a good idea to do this trip after a Santorini→Athens flight, but the Meteora are also very far north from the other places you want to see.

In 5 days you won't be able to visit Delphi, Nafplio and Meteora without spending your vacation behind the wheel.
Save meteora for a future trip

In Nafplio, If you want to have fun with the kids you can go to the open-air cinema. It's a little outside the old town 150 meters after the fire station.
The entry is here:
https://maps.app.goo.gl/CKH7aU6aSbCtWbRK8

They show the movies in English in the original version, there is a bar and an ice cream seller and lots of young Greeks who come to spend the evening

As for the beach, don't go to Tolo, drive a little further to Candia:

https://maps.app.goo.gl/boJQV5DZQGMRwkfc9

I often suggest one look at a map, determine which of their desired stops is more geographically remote and cut that one out if necessary.

You have quite correctly identified Delphi, Meteora, and Nafplio as desired destinations.

I know everyone wants to go to Santorini. It's beautiful (and also a bit of a pain in the rear) but remote.

You can get all of the desired travel thrills if you just land in Athens, hop in a car, drive to Nafplio or Delphi directly and then take it from there. 10-12 days, ending in Athens, drive to airport, drop car, walk calmly into terminal.

We didn't visit Santorini until our fifth month-long trip to Greece and only then because it was Unicorn Season in 2021 (Covid meant the place was largely deserted - I never would have gone there in June otherwise).

All of these detailed responses are why I appreciate this forum! The feedback is wonderful! Thank you!

Santorini will add considerable cost and time spent in route to somewhere to the trip. We hate crowds, so this does make me anxious about visiting this island in June. As I mentioned, we’ve never been to Greece but seen plenty of pictures of Santorini so it’s tugging at us. But your comments and experiences have me swayed. Would a day trip to another island or location be worth a visit if we left out Santorini?

Delphi and Napflio are both excellent and are choices that will give you a real feel for Greece and its history. We did our visit to Santorini but will never return there.

“we’ve never been to Greece but seen plenty of pictures of Santorini
so it’s tugging at us”

It's normal, Santorini is one of the most photographed places in the world. This island of 15,000 inhabitants is visited by around 2 million people per year.
Go to Santorini and take your own photos too. After that, there is very little chance that you will want to return there again, especially if you have visited a few other Greek islands.

Many Europeans only go to Santorini because there is an international airport with flights from quite a few European cities with budget airlines. And the island is quite well located to easily reach other Cyclades islands.

The last time we went to Santorini (2019 I think) the flight from France cost less than €50 and it was only to go to Amorgos which is close.

We went to santorini in May of 2018 with our young adult children. It was not impossibly crowded and we enjoyed our three nights there. We then ferried to Naxos and my two boys took off. They met us for dinner on the beach and they greeted us by exclaiming that “Naxos is even better than Santorini!”

Naxos I understand is more popular than we were there. But it doesn’t have cruise ships visiting. It is worth consideration.