This jewel-like glacial lake (2352m), surrounded by the razor peaks of the Aiguilles Rouges, is usually accessible to hikers from June through October. Two gentle hours from Télésiège de l'Index leads along the western valley to stunning Lac Blanc. There's also the steeper trail from the Téléphérique de la Flégère.
Reserve a place at the Refuge du Lac Blanc, a wooden chalet famed for its top-of-Europe Mont Blanc views.
For a calf-stiffening challenge, experienced trekkers can take the 1050 vertical-metre hike from Argentière (3½ hours one way).