Everyone's first stop should be the Vieux Port, Marseille's beating heart since roughly 600 BC. Its massive facilities are home to working and pleasure boats, and the architecture along its quais are pure southern France. Search out some good bouillabaisse, and have lunch while you check out the scene.
Then head over to Cours Julien, a hip area of town, and see the newest street art; shop at the funky boutiques in La Plane, another cool neighborhood; and spend time in the museum complex known as the Centre de la Vielle Charite.
Cassis itself is a small fishing village that looks like it appeared fully formed from a postcard, and is home to the eponymous liqueur that gives a kir, the popular aperitif, its color and sweetness.
Even if you've been to other markets in France, it's guaranteed you've never seen anything like the Marché des Capucins – unless you've also been to Northern Africa.