Start your hike with quality hiking shoes. Gortex® uppers and Vibram® soles are the best. They allow your feet to breathe, wicking away moisture. The sole is rugged yet very flexible. Don't wear the shoes you bought yesterday or on the way to the trail! Wear them for a few days to make sure there is a good fit and your feet are comfortable. Blisters, halfway through your hike will ruin everyone's day.
Bring only what you need or may reasonably need. A backpack is still recommended but a day trip means a lighter load. No need to carry a tent or sleeping gear but either wear or carry clothes that will keep you warm and dry in the coldest temperature expected in the next 24 hours. As temperatures rise, put the excess in your backpack.
Dress in layers. Temperature will change as elevation and time of day changes. A light waterproof jacket serves triple duty for wind, rain, and cooler days. Easily change into shorts with hiking pants that have zip off legs.
Choose an established, well documented route for your day trip. Keep your hiking distance within your limits. Remember that on many trails you will have to hike back to your starting point. With a good route plan, and knowing your limites an unexpected overnight stay on the trail is much less likely.
Know your trail, know your limits.
Even a "dry" trail may have puddles from overnight condensation or previous days rain. Breatheable water proof shoes help. Dry socks in the back pack are worth carrying too.
Shorts and no hats...DO remember the sunscreen!
