A fantastic park, which must not be missed, is Sequoia National Park. In that place you can find the biggest trees in the world, concerning the diameter of the base of those trees. The Redwood trees, which are related to the Sequoia trees, are even taller but not as big as the Sequoia's.
The trees in Sequoia National Park can reach a height of 80 yards and more and have an average base diameter of probably 5 yards. The park staff gives you even higher numbers but I honestly doubt it. Unless they are measuring the diameter right at the ground (almost at the roots of the tree) those numbers seem to be slightly too high.

Anyway...it's still an impressive sight! The easiest way to see the trees is, of course, to drive through Sequoia National Park, but I would recommend to take at least a short hike through the woods. That way you'll really feel, how powerful nature can be...

Arches National Park / Bryce Canyon National Park / Canyonlands National Park / Capitol Reef National Park / Crater Lake National Park / Death Valley National Monument / Grand Canyon National Park / Lassen Volcanic National Park / Las Vegas / Lava Beds National Monument / Mesa Verde National Park / Monument Valley / Mount Rainier National Park / Olympic National Park / Petrified Forest National Park / Point Lobos State Park / San Francisco / Salt Lake City / Seattle / Sequoia National Park / Valley of Fire State Park / Yellowstone National Park / Yosemite National Park / Zion National Park