Pretty much everyday of the week, there is a marche in Aix-en Provence. Everyday seems to have a special theme, such as clothes, jewelry or special souvenirs (honey, lavendar). The price is pretty good, and it is always nice to walk around the markets and always end up finding some interesting things.
On Saturdays, there is a big market, which seems to be a combination of all the markets that opened that week. This is a huge market, with lots of people, and many things to buy!
The markets are a great place to look for souvenirs, spend time, practice a bit of French and just enjoy the French culture.