Nice is a great base camp for exploring the Cote D'Azur. Buses are a convenient way to visit the neighboring towns of Cannes, Monaco, and St. Tropez. You can even pop over to Italy for a day trip.
Avignon makes a great base camp for some of the smaller surrounding towns in Provence; the train station is right outside the town walls and walkable from almost anywhere in town, although you may find it easier to just rent a car (rental cars are also available at the train station).