Crete. and Rhodes as well, are not as crowded because they are not as "famous" as the Santorini and Mykonos. Also lesser cruise ships call on Crete or Rhodes while every ship calls on Santorini and Mykonos.

Athens worth a couple days even just for the Acropolis if you haven't been. Outside town by the sea there is a lesser known temple ruin, can make a pleasant day trip. The archeology museum is well worth a visit, even with kids. They have done a great job to showcase their treasures. I much like the enlarge projections on the wall to show the small details on the sculptures that even you are standing right in front, you may miss those. Also it is very fascinating to see where these treasures are discovered, not to mention the famous Mask of Agamemnon.
https://en.wikipedia.org/wiki/Mask_of_Agamemnon

You may enjoy 2 to 3 days stop at Athens. This way you can make the journey less tiring and more enjoyable.

But if your goal to Crete is only for the Cat 7 properties, then the above may not work for you. Still, it probably would be better to make use of the trip seeing more new sights, discovering more new things, then end it with the relaxing days by the sea. just a thought.