One of the easiest Calanques to access (part of the Calanques National Park). Hop on bus 21 from Marseille to the Luminy university campus and head off on the trail. Good walking shoes, water, sunscreen are recommended.
Nice (long) walk in nature, before arriving at an isolated beach. The calanques are a series of cliffs (like fjords) with remote beaches. You can access them by boat (from the Old Harbor in Marseille or from Cassis), but it is much nicer (but more work) to walk there. This calanque is one of the easiest to reach from Marseille.
Bring food, water, good walking shoes.