What are essential items in an emergency kit?
Another question asked about emergency supplies you'd bring on a multi-day hiking trip. But many people prefer going on shorter day hikes (say, leaving in the morning and returning to the trailhead in the late afternoon) where it's less practical to bring a large backpack with every conceivable emergency supply.
Besides appropriate clothes and water, what equipment would you consider essential to bring on a day hike (for safety, comfort, protection, etc.)?