The Aiguille du Midi cable car is the single most spectacular thing you can do in Chamonix. In two stages it carries you from the town centre at 1,035 metres to the needle-shaped summit station at 3,842 metres, rising nearly 2,800 metres in roughly 20 minutes. The views from the top terrace are staggering: Mont Blanc fills the sky to the south, the Matterhorn is visible in Switzerland on a clear day, and the glacier far below makes the scale of the Alps suddenly tangible.
At the summit, a glass-floored "Step into the Void" cubicle allows you to stand above a sheer 1,000-metre drop — entirely safe but reliably terrifying. There is also a ridge walkway between the two summit pillars, a restaurant, and a small observation platform facing Italy. In summer, experienced mountaineers use this as the starting point for the Vallée Blanche glacier traverse into Italy. Book tickets well in advance in July and August — queues can run to hours without a reservation.