What's the nicest town or village in Provence for a stay in September?

Provence is very varied. Do you want to be along the coast or up in the hills in Hautes Provence? Do you like to hike?= or want a beach?

You refer to La Provence?

I would love to spend some time in St. Paul-de-Vence again. Eat at Colombe d’Or, under the Picasso mural, visit the Maeght Foundation art gallery. Go to the nearby Matisse Chapel.

I found Avignon to be very nice.

If you want a low-key but artsy town by the seaside, try Cassis. For a typical village in the Luberon (hills), try Bonnieux.