I had a nice experience with Norwegian Cruise Lines last year when I did the Mediterranean tour. Had stops in Spain, France, Italy, Greece, Malta and Croatia. It was a 12-day cruise and 0-days out at sea. Italy, France and Croatia were the best. If you like the game of thrones series then go to Croatia as there are two locations there in Split and Dubrovnik. Dubrovnik is very touristy but I loved the city. Split is an awesome little city in Croatia probably my third or fourth favorite city on that cruise. Lots to do in Split and a lot of little shops if you like to do some shopping that is. The tweens may like Croatia as the beaches there are really good. There is also a waterfall you can hike to.
I think you can also do Italy just using the local train line. Milan, Venice, Florence, Rome and then Naples. You would either have to take a cab or bus to Pisa. But I think that is doable as it is a short ride to Pisa from Florence. Rome alone you can do about 15-20k steps in a day easily just by going to the Vatican. I would suggest at least 2 days in Rome so you can see everything. Pompeii is also an easy 20k+ steps in a day.