Will be in France late May for sixth visit with rail pass. Paris for a few days.
Wanting to visit archaeological sites as Gavrinis in Brittany near Cannes and Nantes. Need rail station, hotel, ferry and possible car suggestions.
Same for Sarlat to see Lascaux and caves.
Going on to Avignon afterward. Thank you all.
Paris to Gavrinis island is possible with pubic transportation, taking the train and changing in Vanves. You have to go on a guided tour (in French) from Larmor-Baden. So Paris-Vanves-Larmor-Baden-boat/tour.
After that it's trickier to reach your destinations by rail. Vanves to Sarlat is 10 hours by train or six driving. Sarlat to Avignon is nine hours by train with three changes, one of the trains running only once a day, while driving is five and a half hours. The problem is that trains in France are designed on a spoke system, radiating from Paris. The railroads were individually built as private companies, starting and ending in Paris. So your route isn't accommodated easily by train.
You can check the travel routes on the very reliable rome2rio.com.
You can check the driving on the equally reliable viamichelin.com.
Try autoeurope.com for renting a car in one city and dropping in another.
Autocorrect put Cannes in your question rather than Carnac. You may want to change that.
I'll let someone else comment on hotels as in my older age I go for simplicity over charm when toodling around France: Ibis chain 2-star hotels or Logis de France 2 or 3 star (individually owned and can be charming).
For the Dordogne you'll really do better with a car. You could rent in Sarlat or the nearest station, or drive down from Brittany instead of training through Paris (and changing stations there). Likewise on to Avignon.
Sarlat has lots of lodging options, but we stayed at Le Petit Versailles in nearby Beynac and were very happy with it. Again, though, best with a car.
Lascaux II is well worth seeing. Even though it's a reproduction, it's a very faithful one, using the same natural pigments the original artists used. Better lighting and better footing than they had. We also liked Font-de-Gaume very much, along with the prehistoric museum nearby at Les Eyzies. I haven't been to Peche Merle but I've heard great things about it on this forum and elsewhere.
Hope this helps some. Maybe you already have your rail pass, if not I'd do this trip (outside Paris) by car.