I have seen much of the area from La Rochelle to Mont St. Michel, BUT I was with a friend and in his car.
Also when you say, Pays de la Lois, do you mean the Loire Valley area? If you do, I would suggest a train ride to Angers to see the Chateau d'Angers, the old part of the city and a couple beautiful gardens. Would make a nice day trip via train from Nantes.
If you like beaches and such, you might consider some time at Ile Noirmoutier. You might be able to rent a bike and see more of the island and there is a bus from Nantes. http://www.vendee-guide.co.uk/ile-de-noirmoutier.htm
In Nantes there is an old chateau or fortification along the river and the cathedral of Nantes is worth touring.
There are many sea side towns along the sea going to the Brittany area that are beautiful and of course more beaches.
Pointe du Raz is along the sea and might be of interest. https://en.wikipedia.org/wiki/Pointe_du_Raz
There are many small villages in the Brittany area that are nice to view and be sure to have some Crêpe and cider.
If you can I would also recommend Mont St. Michel. https://en.wikipedia.org/wiki/Mont_Saint-Michel