An emery board can sharpen more than nails.
Level of difficulty (1 being easiest, 10 being hardest): 6
It's a given that you're going to need a knife or some sort of blade to survive in the wilderness. There's no telling what you might be using that blade for, but you're definitely going to need it to be sharp. Bring along a lightweight emery board to sharp your blade, and your nails, because, hell, what else are you doing out in the woods?