You haven't said when this trip is taking place. Fall or winter are much less crowded in Cassis--but also some shops and restaurants are closed until spring.
Peter is absolutely correct that Cassis is very crowded in summer. Everything is concentrated into a bowl on the side of the mountain that ends at the sea. You could see the Calanques from Marseille, but if you went to Cassis, OTOH, you'd see the spectacular Cap Canaille overlooking the town, as well. The town is very nice and the setting spectacular, when it's not overcrowded.
Otherwise, there is plenty to do in Marseille. Be sure to visit the St. Victor Abbey and its crypt. The other sites/sights are listed on the Marseille tourism website. I appreciate Marseille, having spent many months in the area.