Island of Delos

If memory serves correctly, there are small ferries that anyone can use that go to the island. If that is still the case, you should be able to do research on your own, hop on an early ferry, stay as long as you like, and take a late ferry back.

We did as poster TC explained.

There is a ticket booth next to the pier at the old port where the ferry leaves to Delos where you can your tickets (this does not include entry to the site).

You can also book here:

https://www.delostours.gr/en/tours-and-itineraries.html

Thanks for all the suggestions.