Marseille is in Provence. Provence is a large region in southern France. It would be best for you to look for the towns and cities near Marseille that might interest you. Aix-en-Provence is a nice town very close to Marseille. Also Les Baux, Arles, Nimes and the Pont Du Gard. Try googling "day trips from Marseille" for ideas, or check out the RS guidebook for Provence or France. Just be sure you don't get back too late for your ship. Does your ship have any outings in their schedule?
You might enjoy spending the day in Marseille. It is a great city, one of the oldest in France, founded as Massilia by the ancient Greeks, home of bouillabaisse (sorry, Nice!). It has the Chateau d'If (featured literary prison of the Count of Monte Cristo), Notre Dame de la Garde, Cathedrale La Major, the Museum of Mediterranean Archeology, the Fine Arts Museum, the Vieux Port, boat trips to see the calanques........and lots more to do and see.
Whatever you decide, there are day tours available in and around Marseille. You might check at TripAdvisor for tours. I, alas, have no personal recommendations.