If you only have a short time in Barcelona, do the Hop on Hop Off bus tour (4 hours for 2 routes). If you have more time, there is so much culture and sites to see in Barcelona. The main ones are the La Sagrada Familia (line takes 40-50 minutes), Parc Guel, Picasso Museum (line takes 30-45 minutes), Las Ramblas, Market area on Las Ramblas south of Placa Catalunya. Will need to take taxi to/from port (5-10 euros). There is a taxi line at the port and at the Columbus Monument on Las Ramblas making it easy to catch a taxi. This area is known for pick pockets but if you're a smart traveler, you won't have any problems. There's also good police presence on Las Ramblas.
We booked excursion on our own 1 month prior to sail date with yourtourinitaly. Our guide was awesome. He accommodated our last minute request for local food and wine and for olive tour. We drove up the Amalfi Coast and he stopped to let us take pictures. We went to Positano for shopping on our own. Then, to a local olive farm for a private production tour and to buy olive oils. Then to Pompeii for walking tour. The guide was great at avoiding the cruise ship tours. The food and wine was awesome at Fratoria La Tagliata--you have to eat here.
You have to book an excursion because there's nothing to do in port town and it's difficult to get our port on your own if you want to see a lot. We booked excursion on our own 1 month prior to sail date with yourtourinitaly. Our guide took us to many places in a short amount of time. The Colosseum (time budgeted 30 minutes), San Paolo fuori le Mura- had the best coffee here (10 minutes), Imperial Forums (10 minutes), the Vatican (need minimum 3 hours), Trevi Fountain ( 10 minutes), Pizza at Pasquino Restaurant (40 minutes), plaza near Pasquino ( 10 minutes to buy art), drove through designer shopping street. We ran everywhere but our guide showed us so much in one day. I can't believe how much we got in one day and we still didn't get to see everything.
We booked excursion on our own 1 month prior to sail date with yourtourinitaly. We had coffee at Bar Duomo across from Pisa (budgeted time needed 15 minutes), Tower of Pisa (40 minutes), Statue of David ( stood in line for 20 minutes with reserved tickets, needed 20 minutes to get through museum), Pizza to go at Pugi near statue of David, Gelato at La Carraia Gelataria (15 minutes), Market of San Lorenzo (2 hours needed) buy leather goods in the store not at the open market, can also fine better shopping if ask for shopping tour, Ponte Vecchio (5 minutes), Piazzale Michelangelo (20 minutes), a wine/olive oil store (10 minutes), stopped along the way for scenic pictures. And again, we still didn't see everything. It was very rushed but got to a lot of sites in one day.
Access to Cannes port is by tender boat and will need to make reservation for early tender boat departure a couple days before Cannes/Nice stop or the day before on TV menu in state room. If leaving on late tender boat (after 10-11am) then no need for reservation. For Cannes, relax laying out on small sandy beach at port entrance and walk around or take the Little Train Tours of Cannes. Or start out early and walk 15 minutes to train station to take train to Nice Ville (10 euros) and tram rail (2-3 Euros, 5 minute walk from train station) to city center to get to market areas. Don't eat in plaza at Nice across from beach; eat on side streets. Wear bathing suit to layout on rocky beach (no sand). Waves crashing against rocks is very soothing in Nice. Need 2 hours to return to ship from Nice on train. Beaches in France are topless (yay).
In Marseille take ship shuttle ($12 USD round trip) to Veux Port town early (8am). Can book ship shuttle as you get off the ship in port and use your room key to pay. Once in Veux Port, Then take the Little Train Tours to Notre Dame (7 Euros) then have lunch around plaza with carousel or go shopping at designer stores. In Marseille port you only have until 3pm to see the attractions. Souvenir shop at end of strip to get back to port shuttle or around plaza with carousel.