Hello waterart; since you posted in the Princess forum I assume that is the line you are interested in; it's also the line I am most familiar with.
Princess has added five or six new itineraries to their already fairly extensive Mediterranean catalogue, including some that combine the Greek Isles with other Med and Aegean ports.
You don't say how many days you are interested in but they have everything from 7 to 15 days. One in particular that looks really nice to me, the Connoisseur's Mediterranean, is 12 days on Royal Princess, one of their smaller ships, and includes, among others, St. Tropez, Portofino, Santorini and Valletta, Malta, one of my favorite places in the Med.
But get a brochure and sit down with your Dad and see what looks good to the two of you. It's wonderful that you are able to spend time with your Dad.