There's a train station on the edge of the town. For practical purposes, much of the city is quite a distance from the train station. It is walkable from the station to the city, just be prepared for a hike. By car, be warned that some of the roads around Cassis have many sharp bends.
The beaches (remember your swimming costume). These can get very busy in the summer months though.
The nearby Calanques (see the Marseille entry for more details).
The boat trips to the Calanques are definitely worthwhile. A choice of trips are available, either tours or drop-on or pick-ups from the Calanques.
There are plenty of souvenirs available, ranging from the tacky to the upmarket. Some of them can be quite expensive so do shop around.
The seafront is lined with seafood restaurants.