Arles is a sound choice, consider renting bikes from 1 Veloc Arles to see the Camargue, Baux de Provence, the Via Rhona…

I would still prefer Avignon to Arles, but it doesn't really make much difference other than the fact that it is far easier to organize day trips on buses from Avignon than from Arles, such as the Pont du Gard and things like that.

Colmar over Strasbourg? To me, Strasbourg has so much more to offer. I view Colmar as a kind of "Disney" ville; so many tourists, so small of a town. Maybe worth a half-day trip from Strasbourg (about 30 minutes by train as I recall).

Actually, that was my opinion as well about choosing Colmar over Strasbourg, but I didn't want to say it since I have the reputation of being a curmudgeon here.

I totally agree. I think Strasbourg has much more to offer over Colmar.