I don't think there is one thing that is ideal for everyone. A lot depends on when you go there and what your tastes are like.
If you are really on holiday and not between business conferences, I would suggest you should relax and take advantage of the local pleasures. You can easily do this without an auto. In other words, I would concentrate on the relaxing and enjoying rather than the travelling and seeing.
If you will be there after August, you might spend half the time in Aix-en-Provence and Avignon and the other half on the coast. Personally, I prefer Nice over Marseille, but that is my personal taste. There is a local train line that follows the coast and stops in most villages. You could easily travel from one village to another with this train.
If you enjoy beaches, you could spend a morning in Menton, have lunch somewhere, then walk to Italy and swim in a beach there. It is very relaxing and enjoyable.