Water and Food
Water is a definite essential, as is a way to purify water you find along the way. You can go with water purification tablets or chemicals, or invest in a small water bottle or other purification filter. Even if you're heading out for a short hike, bring along a few energy bars, nuts, fruit and other snacks that will keep your energy high and stomach filled. Longer hikes and overnight stays will definitely merit more food, and may include such items as canned goods and coffee.
Map and Compass
Since you'll want to know where you're going, trail maps and a compass should be part of your gear. Maps specific to the area, and available at many visitor centers, can point out different trails as well as accommodations. Compasses help ensure you stay on track, especially in unfamiliar territory.
Basic first-aid kits should at least contain bandages, gauze, peroxide and aspirin. You can pack your own or check out the mini first-aid kits available at the sporting goods store, so you have essentials on hand.
Tools and Light
Swiss Army knives, or another multipurpose tool, are a must for your pack, as is a flashlight and matches. Different utilities on the multipurpose tool can give you the means to do everything from opening a can to removing a splinter. Flashlights, extra batteries and bulbs should be in your pack. Flashlights can be of any size, as long as you remember you'll be carrying it. You can also opt for a lightweight, battery-operated headlamp that is worn like a headband and leaves your hands free. Matches in a waterproof container and some type of fire starter will heat food, water or signal for help.
Rain, Cold and Sun Supplies
Your back should have supplies to deal with any type of weather. At the very least, a hooded rain poncho and an extra layer of clothing should be in your pack. Ideal extra layers include lightweight long-sleeved shirts or jackets, pants and socks. Pick fabrics that wick moisture from your skin, like a polyester blend rather than cotton. A hat is another necessity, both to stave off any rain and protect you from the sun. Also make sure to pack sunscreen with a high SPF. You are not out to get a tan, but to avoid sunburn.
Those embarking on lengthy backpacking treks that include camping and sleeping outdoors should bring added supplies. A sleeping bag, tent and packable stove may be on your list to give you shelter, coffee and hot food. Consider that everything you bring will be on your back for hours, so go with gear that is lightweight and easily condensed. Mummy bags, which are long, thin, hooded sleeping bags, pack up well. Tent, stove size and type depends on how much your backpack can hold or you are willing to carry.