After Paris, I was planning on taking the TGV to Lyon and spending two nights. As I research, I'm thinking two days may be too short a time to appreciate Lyon and I may miss places I'd like to see in Provence. I already have 6 nights booked in Arles,

If I skip Lyon, I'd spend two nights in Avignon. One of those days, I'd like to rent a car and try and see Orange and Vaison-ls-Romaine in the same day. Are both places in a single day doable?

2 nights may be 1 fay or 2 days. If you visit the Lyon museums, Fourviere and nearby Vienne, 2 days may not be enough.
On the other hand, Avignon and surroundings offer stuff for a whole week (Avignon, Orange, Nimes, Arles, Pont du Gard, St. Rhemy, Glanum, Les Antiques, Les Baux, Aix-en-Provence....

I will be in Arles for a week in mid-May. I will try to post some recommendations after my trip. I am flying home from Lyon so will have a little time there as well. When is your visit?

I'm planning the first two weeks in October. I've been to Provence before (Arles, Le Baux, St. Remy, Pont du Guard)but there is so much more I would like to see. I'd take an early train out of Paris and be in Lyon for two full days more or less.

Two days is enough to scratch the surface in Lyon, but obviously you could spend more. It's a lovely city with great transportation and many great sites.

A one-day car rental in Avignon will cost you about the same as a 3-day rental - why just one day, when there is so much to see in the area and it's not that easy getting around on public transportation?

I've already got a 5 day rental scheduled in Arles. The one day is if I skip Lyon and spend to nights in Avignon.