We recently spent 5 nights in Aix and discovered a couple places worth visiting (and not mentioned in the guide).
First, Camp des Milles, a 20-minute bus ride from the center of Aix. It's the only major internment and deportation camp in France that is still in tact and open. It is profoundly moving. More than 10,000 were interned at this former tile manufacturing plant during the Vichy period. The museum features three sections: The History Area, focusing on the Holocaust, and stories of some of the people who experienced this horror, and some who resisted. The Remembrance Area, the actual parts of the building where the prisoners slept and hid, as well as artwork prisoners (including Max Ernst) did during their internment. The Reflection Area, which is primarily in French, where visitors are encouraged to understand the behavior that allowed these atrocities, and how to resist it. History lives on at this site, developed by UNESCO and opened in 2012. Don't miss it.
Next, the Musee Granet, and Musee Granet XX (in a former chapel). These two buildings house wonderful art collections, for one ticket price, ranging from Cezanne; his student, Charles Camoin; and Picasso, to much more modern works in the Granet XX. The museums are about 2 blocks apart, in the city center, and are a manageable size. Art lovers should plan to spend a couple hours here. It was such a treat to discover wonderful artists who were new to us!