My daypack has the following on every trip:
I would add to the above:
Hat with a good brim--shades the eyes and keeps the ticks off your neck, sun and rain off your noggin.
Jacket or a wool sweater
Poncho or even a compact, folding umbrella (helps with the sun, too), and the poncho can help to make a temporary shelter if need be.
A couple of spare pairs of socks
Cell phone--leave a message with family or friend telling where you're going, use it if you get lost (assuming there's service)
A disposible lighter for when the matches get wet