Barcelona to Southern France - suggestions

I would start with something like this, but avoid taking a rental vehicle across the Spain-France border.
Barcelona - 5 nights
Costa Brava - 3 nights
Provence - 5 nights
Côte d'Azur - 4 nights
Fly or train from Nice back to Paris.

A car rented in one country and returned in another will have a surcharge. I've seen reports in this Forum ranging from €50 to €600 (not a typo - six hundred euros!). However, a car rented and returned in the same country, even in different cities, usually has no surcharge.

So, if you can arrange your itinerary to take a train from Barcelona to southern France, then pick up a rental car there, you will probably save a lot of money. Of course, you have to check the prices for your specific pick up and drop off points, and compare that with the cost of train tickets.

I would recommend spending two nights in Montpellier, it is a beautfiul town and not far from the beach. You can get the train to Barcelona from there. Enjoy your trip!

If you want to have a car throughout the south of France I would pick it up in Perpignan. It sounds like you have plenty of time to make your way to Nice. If you're don't want to see the Riviera and are only going there to fly back to Paris there are other options. We drove through France, ending in Provence, and took the train from Avignon to Paris.

Thanks for the great tips everyone - really much appreciated.