In my opinion, its almost easier with France to state which areas to avoid: the industrial north east, the Landes (ok if you like cycling along straight flat roads through uniform pine forests) and the agricultural plains of central france are a bit dull.
As per Canal des deux mers, if you're completely adverse to hills, you could always try the Nantes to Brest canal and incorporate a trip to Britanny or the western coast, the only downside about these places is that they are more touristy and more expensive along the coast.
I'd also agree with Lewis ref the Jura which could incorporate Alsace, Doubs, Beaujoulais and Bourbonne as regions within reach.
If you're not adverse to doing a few hills (as opposed to low-level mountains) my personal recommendation would be to check the out the following areas: Dordogne, Limousin and Correze: lots of peaceful rolling countryside and woods with a high level of coincidental historical interest and picture postcard villages.
Lonely Planet and Ciccerone guides to cycling France are both excellent if you're still having difficulty making up your mind. Good luck!
