Essential

-water
-food (lunch and/or snack)
-raingear (jacket and pants)
-extra sweater or fleece
-map
-bike pump
-extra bike tube
-tire irons (for changing the tube)
-money
-helmet
-sunglasses

Recommended

-first aid kit (band-aids, alcohol wipes, 2 bandanas or triangular bandages, tweezers)
-padded bike gloves
-waterproof booties to go over your shoes