Renting a car in Greece, or Sanotorni rather is just like renting in Mexico! You get off, you walk a couple blocks and there are rental agencies everywhere!!! I rented 4 quads, lots of fun, I was too small for a scooter! Grab a map and just scoot all over the island, you can do the whole island in 3 hours if you don't stop to look, but since you will, cause it's worth it, make sure you check out the most picure"esque" point in Oia, and then make it all the way from one tip of the island to the other, it's a beautiful view (from Oia to the lighthouse on the otherside)
As for your other ports of calls I've only been to Naples on that list and since you've already done Almalfi, and Pompei, those were the best things to do! However, Vesuvius was good aswell! But not as exciting as Etna in Sicily! Anyways, enjoy and the cruise and as forSantorini, ummmm..... lets see, off the cable car, take a left, then a right when you see the chinese resturant (which is right in front of you), take another left at any opening , you'll end up on the "main street" and there are lots of rental places there. Have fun, Santorini was definitley one of my favorites.