Lot's of great suggestions on this thread.. I'll refine some of what others have said with a little more commentary
there is a great airport with a campground on site in Columbia, CA (O22)
By far, this is the best one, in my opinion. There is a grass runway, a paved runway, gas, sometimes see cool fire fighting planes practicing, and a really great town walking distance to airport, plus a big reservoir for boating and stuff. The approach is also very pretty flying along just the western side of the Sierra Nevadas. 10/10
It's a cool spot, but camping space is limited and you will occasionally get some homeless, or a little "riff raff" from the Pismo beach area. But the beach is walking distance, lots of cool dunes, and the SLO area is cool in general. Only catch it, it's often hazy or "marine layered" in.. so plan to arrive late in the morning and not to depart before 10am
After Columbia O22 this is the second best. Really cool approach through the valley, fantastic camp ground, great hiking and rafting near by. Only cons, not much of a town, not much in walking distance
Awesome spot.. but the weather is often hit or miss
Cool spot but very desolate, and the "thermal hot springs" are just a run down swimming pool.. unless you hike way back into the canyons