If you clicked on this because why La Ciotat for five nights, you might be a local.

Given the utter dearth of references I can find about La Ciotat here or TA, it seems like it's a place people live but maybe not vacation (unless you live nearby).

I'm contemplating a fall visit and a friend will be there with her family and friends and assuming this goes through I'll be joining them for part of this trip.

At any rate, things to do near La Ciotat would be welcomed (if for no other reason it's so hard to find things on forums), but my main question is "where to stay the other five nights".

I've been to this area a few times. I've stayed in Aix en Provence twice before, Cassis twice before. I've been to Arles three times, Avignon and Saint Remy and the countryside nearby a handful of times. Not yet to Nimes, not yet to the Camargue.

I've briefly visited Marseille but only an afternoon from Cassis. I've never been to Saint Tropez or Sanary Sur Mer or that stretch of coast east of Cassis other than driving through to get to Cassis.

If I stay in Marseille, I would plunk in Marseille and spend the five days exploring and maybe not leave.

If I stay in Aix or Saint Tropez I'd have a car and explore. The home base would be for mornings and evenings.

I'm leaning towards staying in Aix with some inland day trips and perhaps one day taking a longer drive and visiting Saint Tropez. Castellet looks inviting too. There are a number of little towns I haven't seen yet near-is to Aix although I've seen a number of the more popular ones.

If I stayed in Saint Tropez I would almost certainly simply explore up and down the coastline. I am not a beach sitter but I do love walking and if there are nice hikes along the coast I'd likely mix that with visiting coastal towns (although we may be doing that from La Ciotat too, TBD).

If you've read this far, any thoughts welcome. Thanks!

Nîmes if you like Roman history and a laid-back, pedestrian oriented town with good links to its region by bus and train, including Uzès, Pont du Gard, Avignon, Arles and Aigues-Mortes.
Marseille for a wealth of visitable sites, great food, lively streets and the possibility of boat tours to the calanques, Chateau d’If, Frioul Islands…and of course Aix, too.

La Ciotat is a very popular beach resort. Everybody in France (and not only in France) knows it.
There are tons of places to visit in Provence, Côte d'Azur and Western Liguria.
But in order to make useful proposals, we should know what you are interested in.

I enjoyed my brief stay in La Ciotat -- charming little harbor and a great base for a boat trip to the Calenques. I've also visited / stayed in each of the other places you mention. If you haven't seen it, I wrote about all of them in my trip report:
A stellar month in southeastern France

KJA - outstanding! How did I miss that?? I'll dive in.

Necker - point taken, I should have said "US based people" because these forums tend to be a lot of Americans and it is barely mentioned in passing. For an American La Ciotat sounds like a hidden gem. It sounds like it might be a bit bigger than Cassis, or maybe more of a year-round town - Cassis is gorgeous and struck me as having a lot of second homes.

FTOttawa - Roman ruins, yes please. Nimes has been on my to-visit list, I just haven't gone in that direction in my other visits.