Are you planning to visit Sicily for just one day? Shore excursions from cruises don't have to be superficial or poorly-planned. There are lots of ways to discover Sicily this year, however briefly. Palermo shore excursions (to Monreale, Segesta, around old Palermo or up the coast to Cefalù) and shore excursions from Messina (with their emphasis on Taormina and Mount Etna) are a great idea if you're taking a Mediterranean cruise that calls at Palermo or Messina. This isn't a week-long tour of Sicily but a one-day outing.
If sightseeing isn't your favorite activity, think about an alternate activity like a Sicilian cooking class. In Sicily beaches would seem like a good idea but those near the large cities are crowded and probably not worth the effort.
Some practical guidelines are in order. Palermo is far more interesting than Messina and the port isn't too far from the historic part of the city, so most of the information you'll need for a walking tour can be otained online from See Palermo. It is also possible to hire a tour guide in Palermo for an interesting walking tour. However, if you plan to go further afield it's a good idea to hire a specialized touring service that provides you a guide and driver. That is also useful advice if you want to get to Taormina or Etna from Messina which is, well, not a very exciting city.
The cruise lines offer bus tours for larger groups but that isn't the best solution for everybody. Hiring your own driver-guide is not necessarily much more expensive, especially if you do it with 3 or 4 other travelers. A few choices are described on our site's Sicily tour page but we're not the only game in town. Whatever approach you choose, enjoy your visit!