A must see on a visit to California in my opinion is a visit to Yosemite National Park. It is just breath takingly beautiful especially in April and May if there is melting snow from the Sierra Nevada, this makes the waterfalls which fall 4,000 feet into the valley wonderful, but don't stand too close to them the water is only a little above freezing point.
After that a trip to Sequoia or Kings Canyon National Parks, to see the Giant Sequoia trees. They must be seen to be believed.
For bird watchers a trip to the Pinnacles State Park is a good idea. This small park is located near Kings City on CA 101. It is a must see for ornithologists because you can see wild California Condors. The State Park is a release point for hand reared juveniles and you are almost guaranteed a view, either of the birds in flight or roosting in trees around the park. You may also get to experience an earthquake too as the park is a very active seismological region, with several small quakes a day sometimes.