Itinerary Suggestions?

When are you picking up the rental car? Better not to have a car for the Nimes part, as Nimes is not car friendly (overnight parking being one issue, in addition to traffic).

But you'll need a car to do the Saturday itinerary.

On Saturday, driving times and not getting lost will be issues to focus on.

For not getting lost, a GPS would be helpful, although pricey for just one day, unless you can get it with the rental car, which (unlike here) is sometimes a hit or miss thing in Europe, checking the GPS box on the online reservation doesn't always work.

For driving times, go to this website.

I think you'll get to see a lot of good stuff this way, but your swings to and from Barcelona will be murder.

You might consider saving Carcassonne for another time and planning a late lunch in Coulloire or Perpignan instead.

A better plan would be if you could arrange to fly home from Marseilles, saving you a day and a half (that you could spend in Provence or overnighting in Carcassonne).

Or fly to Marseilles from Barcelona, renting a car locally, then returning it and flying or taking the train back to Barcelona.

The Provence part is well organized, but it is very active and I think you will be pooped with all that driving from Spain.