Cassis has five beaches, the largest is Plage de la Grande mer, a fine sandy beach. My favorite is La Plage du Bestouan which is small and has rocky areas on both sides of it.
My favorite memory is of the picnic lunches we had. We would visit the boucherie and boulangerie, then find a place by the beach or under a tree for a picnic.
Quick Tips:
The calanques, deep narrow inlets in the rocky cliffs along the coast between Cassis and Marseille, have trails that are are great for hiking and cliffs for climbing. There are also boat tours through the calanques.
Best Way To Get Around:
Cassis is small enough to get around totally on foot. There are places to rent bicycles. Buses run regularly to Marseille, Aix-en-Provence, Aubagne, and other nearby places.