Gap, Briancon or Grenoble via Veynes are well worthwhile, but a long day’s trip. Grasse may be a fairly easy side trip. If in Nice have a look at the narrow-gauge C.F de Provence; you probably can’t fit in a trip over the whole line, but a quick trip on the Nice suburban service might be possible.
On the other side of Marseille the line via Martigues is supposed to be of some interest and on my only visit to the area I enjoyed Nimes-Le Grau-du-Roi. Admittedly, in those days my preference was for obscure diesel worked branches with a bar near the terminus.