Marseille doesn't pose for you the way Nice does. It was founded around 600 BC by Greek settlers from Phocaea, which makes it the oldest city in France, and it has spent twenty-six centuries as a port: goods, sailors, soap, refugees and a lot of arguments came in through the same narrow harbor. In January 1943 the Germans, helped by the French police, emptied and dynamited the old quarter on the north side of the Old Port, and what you see there today is a post-war reconstruction next to a hill of crooked lanes that survived. The city is rough around the edges and proud of it.
It's France's second-largest city, and the only large one with a national park inside its limits: the white limestone Calanques start where the southern suburbs end. That's the case for five days. You get a Mediterranean port with real daily life, a seafront of rocky coves and public beaches, and a day of walking and swimming in the Calanques, all on a city bus ticket. Add boats to Château d'If, the fish stew that started it all and one easy trip to Aix-en-Provence, and the days fill up without any filler.
Day one is the Old Port, Le Panier and the MuCEM, day two Notre-Dame de la Garde, the Corniche and a swim, day three the Calanques from Luminy, day four the Frioul islands and Château d'If, and day five Aix-en-Provence. Start on a Saturday: the MuCEM is closed on Tuesdays, Château d'If and the Vieille Charité on Mondays, so Monday becomes the Calanques day and Wednesday works for Aix. Cassis and the rest of Provence come after day five, and for Avignon and the Rhône side we have a separate guide, 5 Days in Avignon. For the bigger picture, see our page on France.