Cruising provides an economical way to see places like Greece, Italy, Croatia and Turkey. But many people spend that savings very quickly on shore excursions that are overpriced.
This does not mean you should simply remain on the ship and fail to visit the ports of call. It does mean you should usually make your own arrangements either prior to sailing or at the port after you've arrived.
In many places, you'll find guides who are more than willing to negotiate with you on a sightseeing tour. Be sure you set the price (in writing if possible) before starting the tour.
We did splurge on a guided tour in Turkey that took us to the ancient city of Ephesus. The cost was $90 USD/person, and it would have been lower if we had found other people to join us. The cost was more than offset by the expertise our guide displayed in describing the incredible historic sites and cultural attractions we visited.
Dubrovnik (pictured above) was a place where we paid €16/person ($20) to walk the ancient city wall and listen to 24 pre-recorded lectures at various points on rented headphones. It was nice to take this highly informative tour at our own pace, stopping frequently to admire the sweeping, spectacular views of the old city and the sea.
