Rick,
Getting off the ship and onto land will take a bit of time, so you will probably have closer to eight hours, than ten. I like Marseille, but I might choose Aix-en-Provence for my day. It is a rather short bus or train ride (under an hour). It is a lovely university town with a charming centre-ville, a Cezanne atelier, markets three days a week, a flower market, and nice streets for strolling or cafe sitting. A very relaxing way to enjoy your day in Provence.
Marseille has lots to see and do and, in my opinion, its reputation should be much better than it is. It has the Mediterranean Civilizations museum at MUCEM (Marseille was a Greek outpost called Massilia long before the Romans took over), the Frioul islands (and Chateau d'If, the castle with a prison where Dumas' fictional Count of Monte Cristo was imprisoned...you can tour the prison there), the neo-Byzantine Cathedrale la Major (very opulent interior), Cathedrale Notre Dame de la Garde (with bullet holes from attacks during WWII still in the walls, and ship models hanging inside for the fishermen who died at sea), the Vieux Port (fishermen still come in the early morning to sell their fish... think of the movie "Fanny" with Leslie Caron), the Panier district (oldest neighborhood), Parc Borely (gardens), the Vallon des Auffes (another spot where fisherman keep their boats and bring in their catches). And last but not least it is the birthplace of bouillabaisse! (Sorry, Nice, we stick with the claim!)
Cassis or Avignon don't have what Aix and Marseille do, IMO. Avignon is further awa,y, and Cassis just isn't for me. (YMMV)
Whatever you decide, enjoy!