A backpack must contain a few vital essentials. The rest can be up to you whether you want to carry them or not.
Water for 24 to 48 hours, water filtration straw, knife, first aide kit, protein (tuna packs, peanuts, canned meats, protein bars, etc.). Carbs are also a must for energy like candy bars, energy bars, cookies, dried fruits, fruits, hard candy, gum, and soda. If you are traveling with pets; they need extra foods and treats too.