The only Carnival tour we booked on our trip went to Olympia but you are not headed there so I can't help you. We didn't book any private tours, we just did it all on our own. Jumped on trains to Pompeii /Sorrento .. its not hard at all with a little bit of fore planning. Read the "how to's of Italian trians" in the Europe section and you will be set.
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