Another decent campground is in Lake County (CA) off Hwy 20. Lake Pillsbury/Pogie Point. Right where the Eel River comes in to the lake. There are alot of things to do there. Boating, either canoes, kayaks, sailing or even skiing/wakeboarding. The fishing is incredible in the lake AND the river. There is a pretty cool lodge up the shore with restaurant, store etc. but if you just want to be gone away from all that you can too. Good trails for off roading/bikes.
Lewiston & Trinity are great too, Coffee Creek, Trinity Center. Several palces along the Trinity, except for the fact that the hiway is right there. I went on a float trip, flyfishing, with my dear departed dad a few years ago, had a gas. Kangaroo Lake is also pretty cool.
Further up I-5 on the other side of the Siskyou's in the Mccloud area can get pretty wild too...and then south on 89 through Hat Creek area has trophy size trout.
I grew up in Santa Cruz. Let me name a few:
Yosemite in winter - Tiltill Valley,
Bear River Reservoir,
Lake Don Pedro,
Henry Coe State Park
The weather and scenery is some of the best in the country.
Just stay out of the craphole big cities and away from government officials.
I like the area up around Hull Creek and Trout Creek. Miles of OHV trails, fires allowed with a permit, and there are some great views. If you camp off of the trails there are very few people around. I drove my jeep on the trails all day for two days in July and only saw 1 other camp. But if you camp at Hull Creek Campground there are a lot of people in the summer and a lot of dirt bikes running around all day and into the evening. Hull Creek And Trout Creek OHV Area - California Motorcycle and ATV Trails
I live pretty close to Yosemite and it's nice for a day. But I prefer places where I can let the dogs off-leash and the way Yosemite is sectioned off with so many different rules is too much trouble. And having to reserve spots is a pain too.