It would take an entire summer to see everything in Yosemite, but if you have only one day in the valley, what do you do? This is what I recommend.
Start off before dawn in Leidig Meadow and watch the stars give way to the orange and yellow colors of dawn. You will see deer and probably a few coyotes. As soon as it is light enough to see the trail, around 5:30 a.m. in the summer, head for the top of Upper Yosemite Fall, pausing at Columbia Rock halfway up the wall to take in the view, as well as to catch your breath. Arriving at the top two hours after starting off, spend half an hour walking around – go to the bridge that crosses the creek, look up the river channel to see the landscape that gathers the water, then walk to the Fall's overlook and watch the river shoot out over the valley and fall. Notice how the Lost Arrow attaches to the wall, and scan the crest of the Sierra Nevada range stretching along the horizon. Head back down, arriving on the valley floor at 10:00 a.m.
Walk to the base of Lower Yosemite Fall and view it from below. In spring you’ll be pummeled by water hitting the rocks and shooting off horizontally. Hike across the meadow to Sentinel Bridge, pausing to look at Half Dome to the left and Sentinel Rock rising up straight ahead. Continue on to Curry Village, following the path along the river for most of the way. From Curry proceed to Happy Isles and head up the John Muir Trail toward Vernal Fall. It will now be around noon. A short ways up, a bend on the trail has a clear view of Glacier Point and reclusive Illilouette Fall. Shortly after the footbridge with its great view of Vernal Fall, the trail splits with the Mist Trail going left and the John Muir Trail going right. Take the Mist Trail to the top of Vernal Fall and look for rainbows. Notice the Emerald Pool and the Silver Apron just above Vernal, and continue on to Nevada Fall. At the top of Nevada, have lunch in the sun, look carefully at the jointing in Liberty Cap and Mt. Broderick and wonder why the glaciers didn’t break them down and carry them away with all of the fracture lines they have. Notice how different Half Dome looks from the backside. At 2:30 p.m. head back down, taking the John Muir Trail this time with its view of Nevada Fall from a higher elevation.
Arriving back in the valley around 4 p.m., take the shuttle to your car and head for El Capitan. From El Capitan Meadow let the grandeur of this granite monolith overwhelm you. Look for climbers on the rock; they are the colored dots. Drive around the bend to Bridalveil Fall and walk up to its viewpoint. Drive up to the Inspiration Point parking lot and gaze up the length of the valley and take in the wonder. Then drive to Glacier Point, arriving around 7 p.m. to watch the sunset color the mountains in the rose and purple of alpenglow.
If you come in winter, you won’t be able to do everything on this list because there won’t be enough light to start hiking until later, it will get darker earlier, and some of the trails going up the walls and canyons will probably be covered with ice or buried under snow.
And if you're not up for a day stuffed with hiking, or you can't get into the valley before 9 a.m., or it’s winter, then just hike the Vernal/Nevada trail at a leisurely pace. And when you’re on top, take more time to explore the area behind Half Dome. When you come back down, walk across the valley to the Indian Caves. A large flat rock near the main cave has holes worn into it where the Ahwahneechees ground acorns for food. Walk on to Washington Column and the Royal Arches, looking for climbers going up, and visit the grand Ahwahnee Hotel. Another grinding rock is along the trail by the parking lot. If you want to watch deer, the meadow by the Church Bowl is a good place to sit.
A quieter alternative to the rush of all this activity is to find a couple of natural settings that appeal to you (like Happy Isles, Mirror Meadow, the bend on the river by Rixon’s Pinnacle) and stay in each place for a couple of hours, watching the valley change around you as the sun moves over the mountains. Discover what animals and birds call that part of the valley home, and feel yourself drawing close to nature.