FireI'm not suggesting you carry something to cook beef stroganoff with on your lunch break. But it is extremely important to have some ability to start a fire in case of an emergency. I always carry a fire source: waterproof matches or some other emergency firestarter. You'll also want to carry some type of tinder, such as fire sticks, or even cotton balls dipped in petroleum jelly and stored in aluminum foil, a Ziploc bag or even an old film canister.
Extra SocksExtra socks are a must. If you accidentally slip into some water, or you're forced to cross a swollen creek, you'll want to change your socks right away. Besides having cold feet, you're almost guaranteed to take home a few blisters without a dry pair.
GlovesAlthough the forecast might call for a mild day, weather can change quickly in the mountains. Overcast skies, high winds, or light rain can chill your hands fast, especially in the mountains.
Emergency BlanketThrow an emergency blanket in the bottom of your pack. They're inexpensive, light weight and take up about as much room as a pack of baseball cards. Using a reflective material, they're designed to reflect your body heat back to you in an emergency situation. You can also use the blanket to create an emergency lean-to shelter as well.
KnifePreferably a multi-use knife such as a Swiss Army Knife can come in handy in a variety of situations.
Other things to consider include medications, a whistle, flashlight, bear spray, toilet paper, moleskin, sunscreen, ski hat, cell phone and duct tape.
Pack your bag for one of these day hikes.