Water
hat
jacket if there's any chance the weather could turn or if you'll be gaining significant elevation
a snack if you'll be out for more than a few hours
camera
cell phone in case you have a problem and need to call for help
What theox said except I always bring a snack just in case of emergencies (if you get lost, you'd be super happy that you'd packed a couple of granola bars or some trail mix!).
And I don't know where you live, but I've often been hiking in bear or cougar country, so packed a whistle... not only for scaring away possible wildlife, but also as an emergency signal.