Two regions in France with gentle terrain are the Loire valley and the Rhone valley. Both have good train connections which would allow you to easily do a one-way tour. Both regions also have "Eurovelo" routes. Overall, I think the Loire offers more advantages than the Rhone. There's a great network of minor roads with minimal traffic, and there are many chateaux to visit. Terrain is quite gentle. It's also a short train ride from Paris.
While the Rhone valley itself is flat, there are mountains and serious hills nearby, so you would be somewhat limited in where you could ride if you want to avoid hills. There are some nice towns with Roman ruins in the valley. The Rhone valley is more likely to have dry weather than the Loire valley, as well as warmer temperatures.