You’re going to love that itinerary.
in Barcelona, as others have said, definitely see the Sagrada Familia. We actually did a tour with Get Your Guide and it was very good. I recommend a tour, rather than on your own so you get insight into everything. We took a taxi right at the port to get there. It was super easy to do.
In Palma my recommendation is DON’T go to the city beach. We were extremely disappointed as it was very dirty and had things floating in the water. If you want a beach day, definitely go to one of the resorts. We had several people on our cruise who did that and said it was fantastic.
For Marseille, we did the RC excursion to Cassis and loved it. To be fair, we aren’t ones to go on all day excursions. We like to come back to the ship and relax before dinner. So if you’re one to go all day, the Cassis excursion may not be for you, but it was a beautiful city.
Enjoy your trip! No matter what you decide, it’ll be fantastic!