We just finished a cruise, here are are thoughts on some of the ports....
We booked tours through the ship for Athens, Pompeii (Naples), Ephesus (Turkey) - the rest we did on our own. If possible, we chose tours that were no more than 4 - 5 hours, we found these enough for one day, got to the 'main' place and got back into the port. I found most of the tours on the internet to be within $$$'s of those tours on Princess.
Thanks to Tom Ogg's website, we did Florence by ourselves and had a blast. We were the second off the ship - grabbed a cab to the train and were downtown Florence before most of the busses had barely left the ship. When we got off the ship there were tons of taxi's there telling us we missed the train and should book a tour throuogh them, we just said no and got on the train, excellent trip! The train station is right in the heart of all the main attractions you would want to see. Visit www.tomogg.com
I booked three tours on line through VIATOR.com - The Hop On - Hop Off passes for Rome & Florence and then a private boat tour through them for Venice. You do not have to buy these passes in advance of getting there like we did. However, I did find it comforting to know exactly what we were doing in each port. The Hop On Hop Off are double decker busses with ear phones that hit all the main sights.