Route: Yosemite R2R2R (CA)

This is a beautiful beast from start to finish.  Mileage is about two miles shorter than the classic Grand Canyon R2R but it has about 600 more feet of gain overall.  You get to hit both the south and north rim looking down on Yosemite's majestic main valley, you get to go up and down each rim and you get to end at the Merced River for cooling those burnt up quads off afterwards.

This prime version starts and finishes at the Swinging Bridge in Yosemite's main valley, it's easy to find from a car or on foot.  Then it's runner's choice, you have two points to hit - Sierra Point along the north rim (Just east of Yosemite Creek/Falls) and Glacier Point on the south rim.  The route ends at the spot it started at, the Swinging Bridge.

Personally I'd do Yosemite Falls trail first, it generally has more people on it the later in the day it gets and it's a lot more exposed to the sun as well.  Cross the big bridge above Yosemite Creek and make your way out to the metal fence at the very edge of Sierra Point.  Looking out across the valley, you'll see all the highlights: Half Dome, Sentinel Dome, Glacier Point, which is where you're headed next.

Go back down Yosemite Falls trail, filter water at Yosemite Creek if you need, or there's water available at the Camp Four bathroom, just a 1/3rd of a mile round trip detour.  At the bottom of the trail, cross the valley to the Swinging Bridge and head up 4 Mile trail to the top of Glacier Point.  Here you have one of the best view points of Half Dome that you can get.  The turnaround is the top of the trail, but if the crowd allows for it you really should take the tenth of a mile detour and go up to and around the Glacier Point Trailside museum overlook, just east of where you top out.

Jam back down 4 mile trail, get to the Swinging Bridge and you're done.

Admin note: The Rim2Rim2Rim route has the same distance and elevation change as the route starting from the river, however limited winter access to Glacier Point makes it less accessible.