Here are some essential items to take with you in your day pack:
Map of the area and compass
Emergency Blanket or Sleeping Bag
Rain Poncho
Flashlight
2 Liters of Water
Food bars, Granola Bars or other food
Waterproof Matches or Firestarter
Backpacking Water Filter
Small First Aid Kit
Multi-Tool or Knife
Parachute Cord or other rope
Signaling mirror or small mirror
Having these items in your pack will help you deal with unexpected changes in the weather, with getting lost or with finding yourself out longer than expected. Water and access to water are critical, thus having water (and the water conatiner), along with a water filter will help you stay hydrated. A compass will do you little good for the most part without having a map. Always tell others (who aren't on the trip with you) where you are going and when to expect you back. If you get lost, stay put! Lost individuals tend to head off in the wrong direction.
