I have to agree with Hank here. You are going to get more bang for your buck (or Euro) if you are willing and able to do the tours on your own.
With a few rare exceptions, anywhere the ship tours can take you, you can also go see by yourself. The ONLY benefits of the ship tours is the transportation is already arranged and the added insurance that if the tour is late getting back to the ship, the ship is responsible for either waiting for the tour or getting you to the next port.
If the tour you want to go on is a LONG way from the port, then you may consider taking the ships tour and paying the extra money. But most of your ports are close and you can get to the popular places easily.
Another hint, if you can find another couple that want to go to the same place, consider sharing the cost of a taxi. Most charge per taxi, so if you can fill it up and split the cost equally you can save money.
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