First, you have to decide what you want to see, and how to get to it. I suggest you get a copy of Rick Steves’ Mediterranean Cruise Ports. He describes your ports and what there is to see/do in the time you have to do it. And how to do it yourself, and how much it might cost. He doesn’t cover many of the islands, such as Mallorca, Sicily, Malta, and the Greek islands, but they are covered in other of his travel books, and by Lonely Planet.
Guidebooks and websites can tell you what entrance fees might be charged for some of the sites. Some ports you can just walk off, others require a shuttle as attractions are an hour or more away. We were long retired seniors and in new ports, usually booked half day basic tours from the ship. But many places we just walked off and wandered. We took the train to Rome and walked…a lot. With a map. We rode the subway in Barcelona, wandered all over Venice.
it takes research. Join the roll call for your cruise and see what your fellow guests are planning. EM