France: the wine country around Beaune (Cote d'Or) is pretty, lots of vineyards and farms and wineries. Not completely flat, but not too hilly either. Nobody seemed to speak English when I was there except the other tourists, but the locals were very gracious about trying to understand my attempts to speak French.
Austria: The Salzkammergut area outside Salzburg is very scenic. Summer is the off season for lots of lodgings, they make their real money off of skiers. Lots of places to take the train up to the head of a valley and cycle down on cycle paths, which could satisfy the "all downhill" requirement.
The Rhein: Start in Basel or nearby, cycle through the Alsace-Lorraine area to Strasbourg, then continue on cycle paths on the Rhein as far downstream as you care to go. Castles, excellent beer, excellent wine, lots of hostels and other lodging, and once again it satisfies the "all downhill" requirement if you stay close to the river.
If, as you say, you're settled on France but can't decide where, get
Cycling France (Lonely Planet Cycling Guide): Amazon.co.uk: Ethan Gelber: Books . I used the earlier edition when I spent a summer working there, and found it to be excellent.
