Calaveras Big Trees State Park is a great place to go to see the Giant Sequoias, the largest trees in the world. Believe me, you can't imagine how large and impressive these trees are until you see them in person. Calaveras Big Trees is the first grove of Sierra redwoods that was "discovered" by Augustus T. Dowd in 1852. Today you can see the stump (although there's nothing "stumpy" about it) of the "Discovery Tree" which was cut down in 1853; the stump was used as a dance floor at one point.
There are two groves in the park, the North Grove, where the "Discovery Tree" is located, and the South Grove, which contains the biggest tree in the park, the "Louis Agassiz" tree. The hiking trail in the North Grove is shorter than the South Grove, and that is the one I took. However, there are more Giant Sequoias in the South Grove. Even though there is a marked path, make sure you wear good walking shoes.