Mercantour National Park and Écrins National Park would be the top #2 spots, I think there is some amazing hiking to do there. Also, even though I spent almost my entire life living in the suburbs of Paris, I feel like I should know the city better. I have this idea of doing a blog/instagram thing about France & travelling but not sure if I will go though. There is already so much content about it that I'm not sure if I will add anything of value.
Rennes is a nice city, have you been to "Le Mont Saint-Michel", about 50 kms north of Rennes ?