Private tours to Florence can be pricey due to the distance from the port to Florence. We took the train (which passes through Pisa also) but i can understand that with a walker, that may not be an option. There should be a few cars waiting at the port that offer tours that you can negotiate with on the spot. Know your price point you are willing to pay and know your Euro conversion factor. If you can find another couple to share the costs with that can 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