I will be travelling from Barcelona to the south of France in mid-July. I would like to see Avignon, Arles, Pont du Gard, and possibly Nice in about 6 or 7 days (from arrival to departure). I have 2 questions: 1. Is this timeframe reasonable or can it be shortened to 5 or 6 days? 2. Which city should I use as a base of operations since I will be moving on to Paris afterwards? Bonus question: Is there a place closer to Arles that I could subsitute for Nice (i.e beachfront, Rivieraesque)?
(1) You can always shorten things, but there is plenty to occupy a week (or more).
(2) Provence is not very beachy, but you might consider Cassis, near Marseilles. You are right to recognize that Nice complicates this itinerary, on the other hand if your heart is set on the Riviera I'm not sure any place else will do.
Arles make a great base, but with a car (not a bad thing to explore the region) you could stay almost anywhere and there are lots of fine small towns. In any case, don't rule out renting a car for just a few days.
Adam is right about Cassis. It's not a sand beach, but it has a scenic port and breathtaking cliffs. You may also tour the calanques (French version of fjords) while you're there. Take the train to Cassis, then take the shuttle into town.
You can also go to La Ciotat.
I would suggest Avignon as a base because it has a TGV (high speed rail) station, which would make your return to Paris easier. It has access to Arles (train) and Pont du Gard (bus/rental car). You could also go to Nice from here, but as discussed earlier, that stretches the itinerary.
We rented a car and drove to Avignon, Arles, Pont du Gard, all the way east to Gordes, stopping to see an abbey with lavenders and other small hilltowns. I think we spent three days touring by car, however, our base was a little town out of Nimes name Generac. This town doesn't have a pensione. We stayed at a guesthouse (it was an old monastary) in the middle of some vineyard. We walked into Generac and we forgot it was Bastille weekend! Had a blast. Make sure you're in France during the 14th-16th for the festivals. We went to Nice which was quite out of the way. Wish we could have skipped it. Since you're from the West Coast (I'm originally from San Diego, just moved here to IL), the Mediterranean was not a big deal and I found the city to be a bit dirty and the people weren't that friendly compared to Paris! If I were to go back, I would make Arles my home base. Loved it. Good luck