Ngaire, I am officially out of the tour setting business after my only attemt in 2003. On that one 7 day cruise, I had private drivers arranged for us and twelve others for the ports of Villefranche, Livorno, Civitavecchia, and Naples.
I did all the planning and made all the arrangements.

I was a complete basket case after the cruise and was so stressed out during the cruise that it was one of the most miserable experiences that I have gone through.

We missed the Cote d' Azur because the ship could not load the tenders in Villefranche due to rough seas. Called and cancelled our private tour.

We missed Livorno because a ship was damaged and sitting in the berth we were to disembark from, so theu moved our ship to La Spezia, where we tendered in. Fortunately our driver knew and drove up to meet us. Lost about 4 hours due to change, which left us with 1 hour in Pisa and two in Florence, and we almost missed the ship, because we were late getting back.

We made Rome and our driver was great, but some in our group took the wrong exit coming out of the Vatican (our last stop) and we couldn't find them. Our driver finally located them over by St. Peters. Our driver took every shortcut available to get us to the ship and we just made it.

Then we hit rough seas that night on the way to Naples. A ship was blocking the entrance to the harbor the next morning and our captain could not maneuver around it. So we missed our tour of Pompeii, Sorrento, and the Amalfi coast.

I like your idea. Just grab a cab!