By the “coast of France”, I assume that you mean the Med. Coast? When is your trip? The reason that I’m asking is that July/August are the two months when many French (and others in Europe) take vacations, and southern France may be packed with people.
From Geneva, you might consider the folllowing places: Dijon and Beaune, Lyon, Orange/Avignon/Nimes/Aix-en-Provence.
If you want to take a stab at the “Riviera”, consider not going as far as Nice to save time. There are some very nice towns such as Sanary-sur-Mer fairly close to Marseille.
As you head to Barcelona, stop at Montpellier and a tiny village called Collioure, which is just north of the Spanish border.