Will be traveling via train from Lyon to Barcelona in early October. Would like two or three day stop over suggestions. We are considering either Nimes, Arles, or Avignon. Like opinions on which would be he most interesting as to history, architecture, and photography.
Nimes.
Plus it's more central to anything else you might come up with and it has more to do in the evening.
I would also say Nimes. An interesting, very walkable town, and central for day trips in the area. And you can take a bus to visit the Pont du Gard --- history, architecture and photography all in one. :)