Unfortunately a lot of the small towns seem like they will be very difficult to visit without a car. I'll certainly have to come back sometime and spend much more time there (with a vehicle).
Marseille is only on my list if I end up flying in/out of there (it appears to be much cheaper to do so, but I'm still researching that). So, if I fly through it, I'll squeeze in lunch and a few hours of sightseeing before I fly back to Spain.
Pont du Gard, happily, is really easy to do with public transit. I think I can fit that and Nimes into the same day.
Gunz, could you sell me on Arles rather than Avignon? The former did not seem very interesting to me, based solely on pictures and tour guides. What am I missing? Bearing in mind that I'll probably mostly be taking day trips to other towns/cities each day, I was trying to find someplace that will be more fun at night. (Not 'nightlife' necessarily, but low-key bars, affordable restaurants, nice/safe neighborhoods to walk through?)
Thanks for the tips