Cassis is beautiful. Parking is extremely difficult. We were there in September after the buses to the remote lots shut down. After much frustration I just tried odd streets until I found a great spot a block off the waterfront on the west side of town.
The harbor is extremely picturesque and the walk along the waterfront is enjoyable with all the cafes and fishing boats.
We read about the Calanque Provencal Fiords and took one of their boats out to visit 5 of the fiords. Nice boat ride and interesting geology.
A busy tourist location but worth the effort.