Yosemite Half Dome Trail
The most beautiful hike I've done in California is without a doubt the Half Dome.
We started our hike from the Curry Village in Yosemite National Park. From that point, it's a 16 miles round trip hike. The top of the Half Dome is your destination and half-way point. We got up at 4 am and were on our way just shy of 5 am on the John Muir Trail. We passed Nevada Falls while the sun was rising (gorgeous!) and we came back by Vernon Falls (quite a strenuous way down if you ask me, unless you love to go down steep stairs for miles...).
Here's an excerpt from the NPS.gov:
The 14- to 16-mile round-trip hike to Half Dome is not for you if you're out of shape or unprepared. You will be gaining elevation (for a total of 4,800 feet) most of your way to the top of Half Dome. Most would say the reward is worth the effort. Along the way, you'll see outstanding views of Vernal and Nevada Falls, Liberty Cap, Half Dome, and--from the shoulder and summit--panoramic views of Yosemite Valley and the High Sierra.
Most hikers take 10 to 12 hours to hike to Half Dome and back; some take longer. If you plan on hiking during the day, it's smart to leave around sunrise (or earlier) and then have a non-negotiable turn-around time. For instance, if you haven't reached the top of Half Dome by 3:30 pm, you will turn around. Check for sunrise and sunset times before you hike. Regardless, each person should carry a flashlight or headlamp with good batteries (hikers commonly struggle down the trail after dark because they don't have a flashlight). Although the trail is well marked, you should be prepared with a good topographic map and compass and know how to use them.
Here are a few links to help you prepare, or if you're just curious :)
National Park Website http://www.nps.gov/yose/planyourvisit/halfdome.htm
The Trail http://www.everytrail.com/guide/half-dome-hike
Or you can just Google "Half Dome hike" and you'll get plenty of info :)