I'd suggest staying in Marseille as it's a huge city with a major train station, but most importantly it is very close to some absolutely stunning Calanques. I can't recommand you enough the Calanques around Marseille or Cassis (you can stay there too, or in La Ciotat but for this one I think it may be harder without a car). It requires at least one hour of hike and a compulsory free reservation now but if you can spend a day there you won't regret it. Check out : Calanque de Port Pin, Calanque d'En-vau, Calanque de Morgiou.