Aix and nearby recommendations

First, there is so much, that you really should read through the guidebooks at the library to get some ideas. It's difficult to know where to begin. That said, Nice is a couple hours each way by car on the freeway.

Atelier Cezanne! Cezanne's studio was a bit tricky to find, but definitely worth the hunt. Tours in English are at 4 or 5 pm: www.atelier-cezanne.
Cassis is a good quick trip and there are beautiful boat trips in the harbor.

If you want Rick's suggestions, be sure to get his Provence And The French Riviera book. His general France book has much less information on this area. I agree that you need to do more research and see what interests you, as there are lots of "must sees" near Aix (Marseille, Cassis, Arles, Avignon, Nimes, the Roman theatre in Orange, Pont du Gard, the Camargue, Ile sur la Sorgue, Les Baux...). You certainly won't have any trouble filling your week. If you don't mind a few hours drive each way, you could do a day trip to Nice. But unless you have a specific sight in mind there, I'd focus on closer places.

The Friday market in Lourmarin. Cassis. The Luberon villages. If you don't mind driving a little farther, Vaison la Romaine and Pont du Gard.

Nice is about 3 hours each way. The Luberon and the Rhone valley are closer. Marseilles is so close you might prefer to take the train. If you are a Cezanne enthusiast you will enjoy a visit to his studio. Note that it includes none of his work nor even, oddly, his brushes.

If you like Renoir too, visit his home and studio in Cagnes sur Mer which is near Nice. Very worthwhile if you are a fan.

Re Cezanne the city has done a great job, not only making his last studio available but also now providing visits to the Jas du Boufan, the earlier home on the outskirts of town that his father bought when Paul was a young adult. The tour there shows how he actually began by painting right on the walls in the house. It was there he did his early, darker work. They also show you the many paintings he did in back of the house, where the trees and pond still are. The studio tour earlier posts refer to is of the studio on the other end of town, an area which back then was undeveloped. There, he did many of the still life canvases (you see his props) and his huge Bathers canvases (you see the slits he had built in the side of the studio so he could easily move the big canvases in and out.) A short walk up a steep hill takes you to the place he painted and painted the view toward Mt. Ste.Victoire, which you still see from there.