Coastal South of France in late September and October

They sure are, no worries there. It's a nice time to be there but don't plan on getting in the sea.

Any town in particular?
You might get more specific answers on the France forum.

We were in Nice at end of September- of course everything was open.

We were there this past mid-October. Everything was open and it was a great time to visit.

You may find some small museums are closed an additional day of the week, especially in October. It would be best to check each website before you finalize your schedule.

We were just there in early May. We stayed in Villefranche sur mer. Loved it! Not too touristy, nice harbor, plenty to do. We spent 3 nights.

I was there (Marseille - Menton) in mid-March about 5 years ago. Everything was open, quite a few tourists, no crowds (except the Irish pubs in Nice on the 17th (🍺+🍀=😵)

Actually, unless it's been very windy, the water is usually very nice in late September / early October, with the exception of some usual cold spots like the Calanques (and even in the Calanques, I've swum as late as 11th November and no, I am not brave at all when it comes to cold water).

Don't forget the swimsuit!