Aix is encircled by a ring of one way boulevards, following the site of the original ramparts, and complicating driving around the city . Aix is a very walkable city, and is best explored on foot, except perhaps to reach the Jas du Bouffan and Mont Ste Victoire. North of the Boulevard Mirabeau you find "Vieux Aix," where there are many small streets to meander through, squares, cafes, restaurants, small shops, museums and historical sites, full of young people, tourists, students, locals. The Cours Mirabeau, widely appreciated as the most beautiful avenue in Provence, was developed as part of a 17th century urban plan for Aix. It has evolved into the center of Aixoise life, full of bustling cafes, shaded by elegant trees, and located across from 17th century homes of the wealthy and well-placed. Be sure to visit the Cathedral St Sauveur, built between the 5th and 18th centuries, and the 12th century Cloister. There are often concerts in small churches in the area; the Tourist Office can give you details.
If you are leaving from the Vieux Port in Marseille, consider a day trip with Viator to Aix; http://www.viator.com/eap/site/1/6684/tours/3547PRTMRSPT003P