Water is the most important thing you need to carry. Something to snack on would be handy too. Wear good shoes for hiking--tennis shoes are good if you don't have hiking boots, but make sure whatever you wear has cushion and support and won't fall off your foot. Other than that, wear comfortable clothing, preferably in layers so that you can take some clothes off or put them on if you get hot or cold. If you aren't using marked trails, you'll also need a compass/map or other way to not get lost.
Anything else is optional--binoculars, a field guide to identify plants or animals, lunch, a flashlight (to see places where the sun doesn't poke through the canopy), or fishing or camping equipment.
