The fastest way to get to Cassis from Avignon is to take the A7 straight down to A8-A52 before getting onto D559 (Route Pierre Imbert) that will lead you into Cassis.
During our visit to Provence we happened to stay in a place run by local folks who advised us to forgo this route and instead drive south on D52 towards La Ciotat and then take the Route des Cretes - D141 - instead.
On the map it promised to be only 30min longer - but little did we know that we would spend almost an entire afternoon on that route.
Reason: the most amazing views of the Mediterranean and the surrounding gently rolling hills of southern France - capped in the end by breathtaking views of Cassis itself. It is impossible to capture the azure blue of the Mediterranean or the red roofs of Cassis against the rolling hills without traveling this route.