You must walk or bike a gentle mile into the park from Yosemite Valley [1] to get to still, perfect Mirror Lake (hike past end of Southside Dr., about 4 miles round-trip). This small lake reflects the already spectacular views of Tenaya Canyon and the ubiquitous Half Dome [2].
But come early in the season—this lake is gradually drying out, losing its water and existing as a meadow in the late summer and fall.
Links:
[1] http://www.moon.com/destinations/california/yosemite-and-the-eastern-sierra/yosemite-national-park/sights/yosemite-valley
[2] http://www.moon.com/destinations/california/yosemite-and-the-eastern-sierra/yosemite-national-park/sights/yosemite-valley/half-dome