Brightly Colored ArticleAnything brightly colored that can be waved to attract rescuers' attention.
WhistleThe sound of a whistle travels farther and louder and will last longer than your voice. Don't hike without one.
Space BlanketIf stranded out on the trail, your biggest danger is loss of body heat. A space blanket is a lightweight and compact mylar-coated sheet that will keep you warm and dry.
BootsBoots offer traction and ankle support to help prevent slipping and injuries.
First Aid KitAt the very least, carry band-aids and antiseptic.
2 Liters of WaterCarry at least 2 liters of water per person for a full-day hike; more if it's a long hike on a hot day. More is usually better.
Use a Daypack or WaistpackKeep your hands free and unemcumbered; you'll need to use them.
Other EquipmentA cellular phone, flashlight, rain gear, signal mirror, nylon cord, knife, sunscreen, mosquito repellent, and food can be useful things to carry.