Day trip to Marseilles

that sounds do-able depending though on when you're trying to park - I used a taxi or Lyft-ish ride service for both the Longchamps and the basilica because at a busy time of day in a busy time of year it is literally faster to walk than to wait for a spot to open up: getting dropped off was much more convenient and faster, and the driver knows he'll get a return fare back down to the port the moment I get out of the car.

What time of the year would that be? Parking anywhere near the Corniche in July and August is pretty much impossible.
These concerns aside, I would aim for the area around Plage du Prophète for lunch. There's an amazing pastry shop nearby in the Chemin du vallon de l'Oriol called "Bricoleurs de Douceurs", as well as a few small restaurants and a bakery.

Or you could drive much further along to the fishing hamlet of Les Goudes, where the Grand Bar des Goudes is excellent for seafood (closed Wed, booking advised even in off-season).

I think that a boat trip to the Frioul is ambitious in your timeframe; with a car I would be happier driving all the way to Les Goudes / Callelongue and walking about 75-90 minutes round trip on the coastal path to Marseilleveyre, thus getting a first taste of the Calanques. Sneakers with grippy soles needed as the rocky path is quite slick.

I am also not sure that Palais Longchamp is worth it; it is quite a hassle to drive to that area and the museums are second-rate at best. Pretty building, though.
So, I would drive straight to ND de la Garde in the morning (the parking lot is small, beware) then to the Corniche, and back to the Panier area by way of the toll tunnels. Park by the Mucem, and explore the Panier and Vieux Port from there.

You need to be comfortable driving in busy cities, and parallel parking with traffic waiting behind you.
Alternatively, the bus from Aix to Marseille is very cheap and fast, there is good public transportation around the city center, and bus line 83 takes you all around the Corniche too.

When we were in Marseilles we found that the food trucks in the Vieux Port area had great sandwiches. You might also visit the Museum of the Ancient Greek Port. It's small but beautifully arranged.
I don't think there are any bad views in Marseilles.

Avirosemail: thanks for pointing out the parking issue around the basilica and the Longchamps. I hadn’t considered that it could be in short supply. I figured that would be more of a problem around the vieux port.

Balso: thanks very much for the detailed information and suggestion to stop at the plage du prophete on our way to the vieux port. “Bricoleurs de Douceurs” looks delicieux! I love the idea of driving to Goudes and walking along the coastal path. I should have said in my OP that we will be visiting the calanques the following day and plan to do a fair bit of coastal hiking in the Riviera in the following days. I can imagine that that trail is stunning though, so hope the JFK corniche provides a similar taste of it. I also should have said that we are going to the Palais Longchamps soley to see the outside.

Susan D: thanks for the feedback about food trucks in the port. My husband just said last night that he would “be happy grabbing a hot dog” and I rolled my eyes thinking “ not in France we won’t”.

It is, however, ideal for visiting the Palais Longchamps on our way
in, then visiting the Basillique Notre Dame de le Garde. From there,
we would drive to the vieux port via the Corniche JFK, parking at a
lot near the Cathedral de la Major.

I really can't see a car being ideal in this case. Apart from the fact that you plan to use a car to get around in the city, parking will as mentioned be hard to find. It seems much easier to just take the train or bus to Marseille and use buses/trams/taxis to get around the city. Otherwise you risk having to spend a lot of time just trying to find somewhere to park the car.

To specify re. parking: it is very easy to park around the Vieux Port and the Panier because there are many underground parking lots.
Near ND de la Garde is hard, because the lot is small.
On/near the Corniche is near-impossible in beach season but not that bad otherwise.
Around Palais Longchamp, it's the traffic that's the issue. Lots of one-way streets and it is often forbidden to turn left on two-way streets.

All in all, it really depends on how comfortable / used you are to city driving. Otherwise the metro and bus is decent enough and will get you around, with an Uber or cab thrown in for ND de la Garde. Forget about Les Goudes without a car, but if you have another day set aside for the Calanques that's better anyway.

You are more likely to get a Tuna Nicoise sandwich. I don't think I saw a hotdog anywhere.

Mmmm, more likely merguez/frites https://fr.depositphotos.com/stock-photos/sandwich-merguez.html

more more likely a halal grilled chicken plate with pita and feta salad