Amalfi coast or Santorini, Greece

For 2-3 nights, getting to/from Santorini, is not going to give you any 'relax' time. It will be mostly travel.

hey hey empty+nester
you will be traveling during a busy and crowded time during high season. a lot of travelers want to go to the same places you do, which will be packed like sardines, plus santorini to me is out of the question. you will be seeing more from an airplane, train or ferry window. plus the time it takes to travel. since your tour ends in rome why not check the beaches a short train ride from rome.
tripsavvy.com/best beaches near rome.
italy is a big country surrounded by water and coastlines. stay away from the extra crowded areas and enjoy what's near by. check out the weather, it will probably be really hot, get a place with A/C.
another area is lake nemi. italymagazine.com/lake nemi and it's wild berry
a 30 minute train ride from rome, stay at a hotel overlooking the lake. known for its wild strawberry, first sunday of june they have their annual strawberry festival, with a parade thru the town, tasting anything and everything made with the sweet berry. always great to attend an italian festival of any kind. maybe a wine tour to any winery there in frascati, (oldfrascati.com) known for white wine, ceralli.it (bakery/pizza oven) with the "best porchetta". read about nona 94 years old still baking,(lacucinaitaliana.com/nonna rosanna's pupazza frascatana), tinello28.it (pasta making and wine tasting.
this keeps you nearby so you can fly or train to zurich for your flight home.
this is my 3 cents worth and may interest you and your family.
have a great trip and enjoy
aloha

I agree with princess - Flying to a Greek isle will likely take too much time and anyway, Santorini is not a place to relax. And you won't find a quiet area on the Amalfi Coast either.

Consider how you will get to Zurich and that you will want to be there the night before your flight home. Will you have easy access to a reasonable train route? Will you have to spend your penultimate night in a place near an airport?

If you have 3 nights I would go to Ponza island.
https://images.app.goo.gl/X4BpSWcDrs9zCxa26.

With 3 nights and willingness to rent a car I would also consider Mt. Argentario Peninsula (Porto Santo Stefano), less than 2 hr. Drive north of Rome. You can also take a day excursion to Giglio island from Porto S. stefano.
https://images.app.goo.gl/SjcRJ44NnP5E2PzW6

For 2 nights only and no willingness to rent a car, I would probably consider Santa Marinella, between Rome and Civitavecchia, very close to Rome and convenient by train.

I wouldn’t consider Amalfi for anything less than 4 nights.

Go to Positano! Our teens LOVED it! Take the fast train from Rome to Naples, local train to Sorrento then local bus to Positano. I was amazed how easy it was, thanks to Rick’s guide! Stay at the Hotel California. Take a day trip into Capri.

Relaxing by the ocean and Santorini do not go together. You will probably stay up high on a cliff overlooking the Caldera. If you want a beach, Santorini is not known for it’s beaches. Go to a different island if you want to hang out by the water or a beach. Plus it is terribly crowded, mobbed with cruise ship tours.
Paros or Naxos would be much better.

Presumably you have reserved the last night for Zurich. Aren't you treating the continent of Europe as everything a quick SUV drive away? For 2 nights, you really shouldn't go farther than Sorrento, which isn't quite on the Amalfi Coast. The door to your hotel in Priano is another one to two hours of travel beyond Sorrento, depending on traffic, ferry latency, and distance to the hotel. It is getting late to book the Amalfi Coast for May, 2020. Maybe demand will be lighter this year?

BTW, Sorrento has only one, unattractive beach. Most US ocean beaches are nicer than the pebble beaches of the Amalfi Coast, but that's a matter of opinion. Certainly the food is better in Italy! As you implied, Sorrento is pretty busy in a normal May. (I've been there the last week of May.)