For a day trip, you don't need much. The 10 essentials is a good guide to follow.
Bring good footwear (broken in hiking boots and double lined socks)
Sun protection (Hat, sunscreen, sunglasses)
A light jacket in case the weather turns sour
Lunch and trail snack food
Water (a 1L bottle should work out fine. Nalgene makes some good ones)
First Aid Kit (Make sure it has mole skin, gauze, triangular bandage, tape, and wipes)
Compass and Map of the trail (learn how to use a compass if you are unfamiliar)
Whistle (in case you get lost, 3 sharp, distinct blasts signal help)
Backpack (To carry everything in. Make sure it is comfortable, preferably with a waist strap)
Flashlight (in case you get lost and it gets dark, or to look in the bottom of your pack)
