Nothing will ruin a trip to the parks faster than sore or wet feet. Take some time planning your travel wardrobe. Bring comfortable walking shoes, even if you plan to keep walking to a minimum. Bring shoes that are broken in, and if you plan to do some serious hiking, get sturdy boots that support your ankles and wick away water. Early in the season, trails might be wet or muddy; late in the fall, you can get snowed on. The more popular trails are sometimes also used by horses, which can make stream crossings a mucky mess.
Wear your clothing in layers, and bring a small backpack or fanny pack so that you can take those layers off and on as temperature, altitude, and your physical exertion change. Cotton is a no-no in the backcountry; synthetic fabrics are recommended because they dry much faster. Gloves or mittens are useful before the park heats up, or in the evening when it cools down again, even in summer.
The atmosphere is thin at higher altitudes, so protect your skin. Bring a strong sun block, a hat with a brim, and sunglasses. I also recommend bringing insect repellent, water bottles, and a first-aid kit.
Take into account that elevations at the parks are between 5,000 and 11,000 feet; in campgrounds and on hiking trails, you'll want clothing appropriate to the temperatures -- in summer, 40°F (4°C) in the evening, 75°F (24°C) during the day.
