It really depends, CA is a huge place. Some of my favorite areas are also along the coast. Around Mendocino and north. North of Ft Bragg is the Lost Coast. Amazing rain forest country. There is a road into USAL beach and hikes along the coast from the USAL beach camp grounds. You can also connect with the pacific crest trail. Watch out for the 'coons here. If you get up here stay and eat for a while at:
http://www.albionriverinn.com/
Carmel and the Big Sur coast are also great. Andrew Molera SP is always a fun time and awesome beach.
There is also Death Valley, Joshua Tree and White Mountains. The white mountains are near Big Pine and are home to the oldest living trees on earth (Bristlecone Pines). A ton of choices.
Maybe you meant Yosemite not Yellowstone?
Yosemite Valley is always crowded when the weather is good. Great place though. Once you are 2-3 miles in on a trail, you willl see far fewer people, except for half dome. Tuolumne Meadows and the high country are slower.
Lee Vining creek (between Mono Lake and Yosemit high country) has large fish in the pond down near the bottom and smaller stockers on the way up, make sure to follow the regulations. Then you have the entrance to Yosemite and Tuolumne Meadows. There is a decent camp ground along Lee Vining Creek, but it is close to 120.
Bodie is a Ghost town and a State Historic Park, pretty cool, north of mono lake.
Head up th 395 to Bridgeport, great fishing and good tips at th bait store here. You can ask about multiuse places to camp, they know the area.
Further north on 395 to the 108, heading west on th 108 there are 4x4 roads to the north. Across from the Marine Corps Mountain Warfare Training Center is good fishing and you may see some of the latest rotorcraft that the Marines are using. If you camp near here and it is not too cold be prepared for the Corps to run up the hill at about 4:30am with radios and singing while they march. I think they like to wake up the campers.
South on the 395 is gorgeous too, passing Whitney, the highest peak in the lower 48 on down to death valley.
Brian