Aix—Things to do recommendations? Day trips?

Will you have a car?

Do you mean you'll be there for 3 nights so you'll have 2 full days?
In Aix en Provence there is the home and studio of Paul Cézanne and the city of Aix walk. There is also the Musée Granet and the Cathedral. It's a wonderful place for just relaxing and wondering the various excellent markets. There are also day tours available if you are not planning on renting a car: http://www.aixenprovencetourism.com/en/

Aix is a lot about strolling, shopping, and café sitting. Aix's biggest market day on Saturday is quite impressive.

You can daytrip by local train to Marseille, 45 minutes each way. You can see the Calenques by boat ride from Marseille's central harbor. Or if you'd like to spend longer in the area, then you can connect at Marseille to a train to Cassis and stay at least two nights to allow a full day for hiking and swimming.

Yes. I’ll have a car.

Outside of the city center (reachable by city bus) is a very impressive memorial site/museum at the Les Milles transit and internment camp used during WW II. The explanatory material has all been translated into English (I think there's also an audioguide), and it provides a great deal of background on WW II and the holocaust. This site is different from many others in that it has a heavier-than-usual focus on "how did we allow this to happen" from a psychological standpoint. It will take several hours to absorb.

The area surrounding the camp didn't seem to have any restaurants. I think there was at least a soda machine at the camp, but I'm not sure whether any food or snacks are available. Given the time needed for a comprehensive visit, keep this in mind.

Part of the visit takes you down to the basement, which was quite cool when I visited on a very warm day last year. You'll be down there quite some time, so I highly recommend taking a warm layer of some sort.

I believe the camp opened in 2012 and is not mentioned in every guide book.