Day Trips from Aix?

I'd say Luberon, Verdon, and the Calanques.

Luberon:

• Get to Lourmarin, explore for little bit
• Head to Bonnieux, restaurants there have great views for lunch. If it's already lunchtime of course.
• keep going to Roussillon. No white shoes if you intend to visit the former ochre quarry (recommended)
• there should be time left to swing by Gordes, if only for this viewpoint west of the village
• head to A7 near Cavaillon for the return to Aix (~1:15 drive from Gordes to Aix that way)

Verdon:

• head to Moustiers Sainte Marie, explore for ~1.5 hours
• drive along the north rim of Verdon Gorge to La Palud
• drive the Route des Cretes one-way loop back to LA Palud (you must not be afraid of heights)
• keep going east to Point Sublime, stop for the views
• cross the Verdon at Pont de Soleils
• if short of time at this point, head straight to the A8 motorway back to Aix
• if there is still time (unlikely but you never know), drive the south rim road to Aiguines, then south on a very minor road to Aups, and to the A8 near Brignoles

Calanques:

• get to Cassis for a boat tour of the Calanques
• quick walk around the village of Cassis
• drive the Route des Cretes to La Ciotat
• stop in La Ciotat if you'd like, the port area and the old town are quite charming and authentic
• optionally, you could start the day with a hike in the Calanques. I'll give you an easy option and a moderate option in a separate reply.

Regarding the Calanques hikes, I have 2 suggestions.
The park website has helpful maps: https://www.calanques-parcnational.fr/fr/cartes-plans-marseille-cassis-la-ciotat
And you need decent shoes. City sneakers won't cut it, I'd want trail shoes or something with a good tread.

1 (moderate) from Cassis: park at the Presqu'île, walk down to Port Miou and up and down a ridge to Port Pin (super slippery beware), then follow the blue trail SW of Port Pin, gradually climbing until it turns west at the top of the cliffs above En Vau, then eventually loops back towards Port Pin from where you can backtrack.
I'd say 3 hours for this, perhaps a hair less.

2 (easy) is from Luminy near Marseille. Do not cross Marseille to get there at rush hour, drive the long way via Cassis and the col de la Gineste.
Luminy is a campus where you can park. A big trail starts at the southern end of the roads. Follow it until you see the sea, then follow a broad path on your right towards a viewpoint with railings (you'll see them), the Belvedere de Sugiton.
It's just a short 1:15 round trip walk to the Belvedere de Sugiton, and this one can be done in any shoes and is basically flat.

There are many more options but it gets complicated to describe them here and you would need a paper map. Besides, some trails are downright deathly - I used to live in the area and I got pretty scared a few times.

TY Balso, this is very good information. Appreciate the details. I am a bit shy when it comes to heights, didn't used to be in my more youthful days but things change apparently. I will do my best to work the options.