You will probably either drive here from Marseille, or ride the train. Assuming the latter… 58 minutes after leaving Gare St Charles, you’ll find yourself faced with a mile or so walk from the Cassis train station. Pack light, it’s downhill but still it is nice to arrive fresh at your lodgings. Walk around, find an outdoor restaurant table. Seafood and a local wine—time to be both mellow and philosophical. Plan to hike, plan to kayak, plan to do standup paddleboarding. Equipment rentals may be had well into October, but check first.
The thing is, you need to see these fantastical cliffs from the water. A motorboat tour the day before, will nicely whet your appetite for taking your rented watercraft out there.
You’ll wish you had a GoPro with waterproof housing.