I would recommend Cassus which is a beautiful port town 30 mins east of Cassis. If you can get to the Saint Charles train station in Marseille (easiest way is to take a taxi from the airport for around 30-40 Euro), you can get a train to Cassis (very short journey). You will have to either get a taxi down to the port when you get to cassis though or walk it (about 15 mins - probably not what you want to do with luggage after a flight).
Another option is to just splurge on a rideshare such as Bolt directly from the airport to Cassis, that will cost you around 100 Euros though (well worth it if you can afford it, in my opinion).
I strongly recommend Cassis, it is super cute, you can get boat rides from the port out to view the scenic Calanques and beaches. The town is very walkable with great bars and restaurants, a small beach right there in town and just a nice vibe in general. If you want a recommendation for an inexpensive hotel, I can recommend Le Golfe. Rooms are basic, but mist gave views right on to the harbor and it has a decent patio restaurant.
Aix En Provence is great but it’s more of a small city and isn’t super relaxing in my opinion - and it’s obviously not on the water.
A good tool to use for figuring out transport and itineraries in France is Google Maps. You can put in the date and time you will be arriving on, your start point and destination, and it will show you various options when you click on the icon for public transport.
Last thing I’ll say is that a rental car is generally a good idea in this part of France. I’m renting a car from Marseille airport next month and the rate for an automatic compact is around $50 a day.
