When preparing for a mountain hiking trip, it's crucial to pack the following essentials: sturdy hiking boots, appropriate clothing layers (including waterproof and breathable options), a backpack, a map and compass (or GPS), a first aid kit, plenty of water, high-energy snacks, sun protection (such as sunscreen, sunglasses, and a hat), a headlamp or flashlight, a multi-tool or Swiss Army knife, and emergency supplies like a whistle and emergency blanket.