Taking the right items on a hike at the very least can be the difference between comfort and misery. In extreme cases, it can be the difference between life and death.
Start by dressing properly. This usually means layering your clothes to protect against wind and cold but also dressing lightly if out on a hot day. If the trail lacks shade, don a brimmed hat and sunglasses.
A good pair of hiking boots is necessary, though trail runners will work fine on flat terrain without rocks. Sandals or going barefoot are bad ideas. Wool socks work better than cotton ones.
Water is essential. Adults need at least a quart for every two hours hiking. Children need to drink about a quart every two hours of walking and more if the weather is hot or dry. Don’t drink from streams, rivers, ponds and lakes.
Map and compass also are required. Topographical maps work best. GPS is fine, but don’t expect to always have a signal when in remote areas or battery power on a long walk.
A small first-aid kit is extremely beneficial in case of an accident. Make sure it at least has bandages, scissors, medical tape, tweezers, and an antiseptic such as hydrogen peroxide, iodine or Betadine.
A quality trekking pole or walking stick is an excellent idea if your trail gains elevation, is uneven, or is wet. A pole reduces stress on your knees and lower back.
Lastly, insect repellent often is a must. Remember that you can sweat off repellent, so bring it with you rather than rely on one slathering when leaving your vehicle.
Other items you might consider bringing is trail mix for long hikes, sunscreen and lip balm if in sunny or windy areas, and your cell phone (and sans that a watch) to keep track of time and for emergency calls.