I did a lot of research on markets in southern France prior to my 2017 trip. I haven't looked at that information for nearly two years, so you need to Google to verify this list of Friday markets. In particular, be sure they will be operating in March; my trip was later in the year, and I didn't necessarily note which markets were seasonal. Many markets are morning-only, so if you want to hit a market it should be one of your first activities.
Cassis: Flea market at Parking du Marche
Cassis: Provencal market at "les rues et pl. Baragnon". Should have food & other items.
Note: Cassis also has a weekday fish market near the port.
Salon-de-Provence: Small farmers market at Quartier des Canourgues
Marseille itself has lots of daily markets in various locations. Google for current information. Aix-en-Provence has a daily produce market at place Richelme.
Cassis and Salon-de-Provence are both accessible from Marseille by train. I have not been to Salon-de-Provence. Cassis is a very picturesque coastal town. It has a small local museum, but a lot of people go there to take a boat trip to see the Calanques. You don't need to do that since there are boat trips to the Calanques offered from Marseille as well. Besides, since you're on a cruise, I assume you're looking for land-based activities. During peak season Cassis is very, very touristy, which to my mind somewhat detracted from the experience of being there. It's probably not so overrun in March, but also not so festive.
Marseille is not one of my favorite places in France, but the areas on both sides of the port are walkable and interesting. However, I found the MuCEM (North side) very disappointing, and it was my impression that Marseille is not a big museum city; perhaps I missed the best.
I'm a big walker and enjoyed strolling to Vallon des Auffes, a little village south of the port. It is not, however, a good destination for people who like sights of the type one goes inside of. It's just a quirky little neighborhood.
Arles and Aix-en-Provence are both very worthwhile, enjoyable destinations. I'd suggest consulting a guidebook or at least taking a look at Trip Advisor's Things To Do lists to see whether there are sights of special interest to you.
I prefer to use the Deutsche Bahn website to check train schedules, but if you're interested in fares you'll need to use SNCF. You can get to Aix in only 12 minutes by TGV, but those trains go to a separate station about 9 miles from the center of town. I'm sure there are buses but don't know at what frequency. The slower trains (TERs) stop less than half a mile from the Cours Mirabeau but take closer to an hour, which is roughly the time it will take to reach most of the other places I've mentioned.
To some of the potential destinations, a taxi might not be horribly expensive, especially if you found one or two other people with whom to share it. You could do that in one direction and plan to take a train the other way. That's just a thought; I seldom use taxis myself and may be mistaken about the cost.
