Along the  V61 Veloroute from Chamonix to Geneva. This route starts at the train station Saint-Gervais-les-Bains-Le Fayet to the west of Chamonix, and ends at the train station Bonneville for a gentle 25 mile route that is almost all down hill. The direct return train takes ~45 minutes, and as of July 2024 costs less than 10 euros.