Death Valley National Park: Lowest, hottest, and driest National Park. It's also the hottest place on earth, with temperatures in the 130s in the summer.
Manzanar National Historic Site: One of the relocation / detention facilities for Japanese following the bombing of Pearl Harbor in 1941.
Redwood National Park: Home to the tallest trees on earth. Jurassic Park did some filming here too.
Sequoia and Kings Canyon National Parks: Home to the largest tree, by volume, on earth! Go for this alone.
Whiskeytown National Recreation Area: A beautiful and diverse place with mountains, lakes, trees, streams, and more.
Yosemite National Park: Amazing. If Ansel Adams was inspired here, rock climbers are inspired here, and millions of people have been inspired here...so will you!