We spent two nights in Cassis and hiked and boated* to the Calanques, which was great (* we rented an outboard motor boat, but upon departure in the a.m., there was so much fog and chop that my eldest child and I requested being taken back to shore, where we hiked from the port to the Port En Vau calanque while my hubby and two other kids went back out in the motor boat). The hike was SO fun. Crowded (July) but so beautiful. You pass two other calanques before getting to port en vau. We swam there for a few hours and cliff jumped, and then took the motor boat (all of us) back. The return in the boat was beautiful and not at all choppy. Perfect day. Cassis is small, charming, quite touristed, but maybe not so bad in May.
You can hike to the Calanques from Marseille but I believe the trail is longer and more exposed. The nice thing about starting from Cassis is that we could walk to the trailhead from town.