According to the Office of Tourism in Aix en Provence, there are about 7 companies that offer day trips from Aix en Provence. Be warned that all of them are Escorted Tours, meaning that they basically offer transportation and a superficial narration along the way. What's more, they're all quite pricey.
What I thought I was getting was a "Guided Tour," meaning that a certified guide drives you to a destination and personally shows you around. Instead, you're dropped off at a central location and told to be back at a certain time. You're on your own!
Further, some companies are unreliable and may not show up. Make sure that the Office of Tourism or your hotel confirms the arrangements. Even so, the drivers are sloppy with paperwork and may pick up the wrong people instead of you.
Next, some companies may promise "no more than 8 people in a minivan," but you can wind up with a group of around 20--yet, there's no reduction in price.
Finally, keep in mind that tours cannot be cancelled. Best advice is NOT to book far in advance. Book a day or two in advance, through the Office of Tourism.
All in all, while the guided walking tours in town are worthwhile, think twice about the escorted day trips.