The Rhone is also a good choice. We cycled along the Rhone in Switzerland, and that was very nice. You could take a train to Evian, then cycle the road to the France/Switzerland border. At the border the bicycle path starts, and then you can cycle all the way past Martigny and Sion. When you're in that area, you can do side trips to Chamonix/Mont Blanc and Zermatt/Matterhorn. Beautiful!!