Over the past 2 years, I have hiked hundreds of miles of trails in the Southern California area while documenting hikes for this site. The following is a brief description of my standard gear, from the top of my head to the bottom of my feet. Note that I am an equipment junkie, and I typically go overboard being prepared for the worst, so you may not want to haul all this stuff along with you on a simple day hike.
Tilley Hat - This is an awesome wide brimmed hat that the manufacturer claims will last so long you put it in your will to hand down to relatives. I have washed mine several times and it is no worse for wear. I like the vents in the top, the secret pouch for holding some cash, and the wind lanyards for keeping the hat on.
Red Ledge Thunderlight Rain Jacket - This is a great rain jacket that comes with a tiny stuff sack that will easily fit into a day pack.
Medalist MicroFleece Zip-T - This is the greatest article of outdoor clothing I have ever owned. It is light, long sleeved 100 weight fleece pullover that is amazingly warm and compact. If there is the slightest chance of cool weather, this article of clothing comes along.
DouFold T-Shirt - This is a relatively inexpensive, ultra-fast drying t-shirt. Perfect for hot days where you sweat a lot. I like the bright colors, particularly yellow.
Patagonia Midweight Capilene Crew - On a cool day, I will wear the Capilene crew next to my skin, with the DuoFold T-shirt pulled over the long underwear. If things get even cooler, the Zip-T comes out. If the wind or rain comes up, the Red Ledge Jacket comes to the rescue.
REI Sahara Hiking Shorts - Loose fit, lots of pockets, perfect for hiking. In cooler climates, REI has Sahara long pants with zippers that will convert them into shorts.
Thorlo CoolMax Light Hiker Socks - These are great socks that are perfect for warm weather hiking. If the weather is a bit cooler, you might want to go with the Tharlo Backpacker socks.
REI Gore-Tex Leather Monarch Boots - Although this is not exactly a light day hiking boot, it is my favorite footwear for day hiking. I like the fact that I can water proof the leather, giving me double protection when combined with the Gore-Tex.
Suunto Vector Watch - This amazing electronic wizard can do everything but dance. It has an electronic compass, altimeter, barometer, thermometer, stopwatches, much more. I use the compass and altimeter constantly while on the trail. The thermometer is worthless and I have never bothered with the barometer, but everything else is great!
Outdoor Research Flex-Tex Low Gaiters - I hate stopping to get rocks out of my boots, and these gaiters completely solve that problem. I won't go hiking without them!
REI Hiker First Aid Kit - This is a great basic first aide kit, with everything you need for the average on the trail minor emergency.
Garmin Etrex GPS - Most of the time the GPS is dead weight, and a surprising amount of the time the GPS can't get a reliable signal. Nevertheless, this device has saved my butt a few times when I became momentarily disoriented while on the trail.
Leki Trekking Poles - I normally don't use these unless the trail conditions are marginal or the terrain very rugged. I like having the extra security of the hiking poles when a slip could have serious consequences. The poles also take considerable load off your knees on the downhill sections.
Ultimate Direction Trekker Lumbar Pack - This is a great pack that rides on your hips, hanging over your butt. It has two 20oz sipper bottles, and plenty of room for lunch, rain gear, first aide kit, and other day hiking goodies.
Patagonia Lightweight Capilene Briefs - It doesn't make much sense to wear lots of high tech wicking fabrics and still head out on the trail with cotton underwear. If the weather is severe, these Capilene briefs do the trick.
Small fanny pack worn over the belly - I keep my snacks, camera, sun glasses, chap stick, and other items in the fanny pack, so that I don't have to stop and take off the lumbar pack each time I need to take a picture.
Topo map printed on waterproof paper - National Geographic Maps sells both the software for printing the maps and the waterproof inkjet paper. Note that it is important to print the map with latitude and longitude values to be of any use with a GPS.
Emergency Essentials - I always carry the following items in my daypack, just in case things go amiss while in the backcountry.