My wife and I visited several of these on a 2011 cruise. Split and Dubrovnik, just walking around town is great, the latter including walking the walls and visiting the old port. One thing to be aware of, the cruise ships dock some distance outside of Dubrovnik. I am not sure if there is an option to dock closer (maybe with tender). We paid $10 each for a shuttle to and from town on a bus the cruise ship organized--multiple time options. Same for Corfu, just walk the town and we enjoyed the old fortress there too although it poured rain all day during out single day in Corfu. We used Rick's Mediterranean Cruise Ports book and it was very helpful. I don't think there is any reason why you need to take a paid excursion in those stops. We stayed three nights in Sorrento in 2007 on a land-based trip and visited Capri. Capri is nice too, but with limited time I would either stick to Sorrento or take the commuter train up to Pompeii. Here too, no reason to pay for an excursion. You can research online how far the cruise port is outside town, and Rick's book is good for this too. My approach is to only book a cruise were the large majority of ports don't force me to take a shore excursion.