Can you tell us which ports you will be visiting? We can provide better advice that way.
But for the most part, the Mediterranean ports are easily done on your own. Many have public transport (trains and buses) within easy walk of the ports.
Let us know where you will be heading and what your thoughts on excursions are and we can certainly help.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013