Personally, I like to sleep in the same spot for 2 or 3 nights. So if you have decided on BCN and Nice, I'd choose a third location somewhere in- between. I'd recommend looking at a guidebook on the South of France or Provence, and then deciding on what appeals. Do you want to see the Pont du Gard, or Luberon perched villages, or stay close to the coast? There are so many beautiful places, it is difficult to choose!
The car drop-off in another country can be expensive, but compare it to the cost of train tickets from BCN to Perpignan or your first destination in France, and decide if it's worth the cost. You don't want a car while you are staying in BCN.