No matter the destination, there's a baseline list we check before every outing:
Clothing: layers — a thermal base layer, an insulating mid-layer, a windbreaker or rain shell. Never cotton as a first layer, since it gets wet and stops insulating. Trekking shoes that are already broken in, never worn for the first time that day.
Water and food: at least 1.5 liters of water for half-day outings, more for a full day. Energy snacks that don't require cooking.
Protection: sunscreen, sunglasses (radiation is stronger at altitude), a hat or beanie.
Safety: a headlamp with spare batteries (even for day trips — an unexpected delay can extend your return), an emergency blanket, a whistle.
Documentation: ID, and if you have any medical condition, keep it noted somewhere visible in your pack.
At Nahueltrek we rent trekking poles and crampons if you don't have your own — check our shop before heading out.