Yosemite National Park is massive--you can easily spend a week hiking, biking, climbing, rafting and exploring. However, if you only have a day, make the most of it and hit these highlights.
Stop at the Tunnel View overlook on your way in to Yosemite Valley; it’s the quintessential welcome and a great perspective of some of the major sights that you’ll encounter during your day.
Head to Mirror Lake early to beat the crowds and hopefully catch some residual sunrise effects on the reflection of Half Dome and Mount Watkins. If it's later in the summer or early fall, skip Mirror Lake; when the water is low, it's decidedly less impressive.
As you have limited time, consider taking the narrated, two-hour tour, which gives you descriptions and history of Yosemite Valley's famous landmarks. If you're more interested in a self-guided tour, rent a bicycle at Curry Village or use the free Valley Shuttle Bus to explore the east end of the valley. Be sure to disembark at stop #6 to hike to the base of Lower Yosemite Falls.
Pop into Majestic Yosemite Hotel for a look at how the other half "camped" in the national park. Marvel at the views of Yosemite Falls from the dining room and remember how the spray hit your face. If you have time, sign up for the free tour to learn more about this historic hotel.
If you still have a bit of time, stop into the Yosemite Valley Visitor Center for an overview of the park's natural history and geology; peruse the Ansel Adams Gallery to view his many photographic love letters to the area; learn about Yosemite's Native American history and culture at the Yosemite Museum.
End your day at Glacier Point. A popular spot for sunset viewing and stargazing (Theodore Roosevelt and John Muir camped here), Glacier Point will provide a picture that serve as the ultimate memento, spanning Yosemite Valley, Half Dome and the majesty of the Sierra Nevada.