Take the van north across the Golden Gate to Point Reyes for the weekend and sleep in the redwoods at Samuel P. Taylor, because nothing camps inside the seashore itself. The redwood loops go on ReserveCalifornia and fill on summer weekends, so book the site first and the van second. You spend the day out on Tomales Point with the tule elk and drive Sir Francis Drake to the lighthouse at the end, then come back over the ridge to the campsite. That's a van trip, not a big-rig trip.
Winter adds the elephant seals hauled out below Chimney Rock and gray whales off the point. Leave the rig at the Bear Valley lot and walk or shuttle from there, because the lighthouse lot is too tight for a van and closed to RVs and trailers.
The Marin Headlands and the Golden Gate
Closer in, drive up into the Marin Headlands for the Golden Gate view the night you have the van. Battery Spencer and the pullouts along Conzelman Road put the towers at eye level, with Rodeo Beach at the bottom of the hill. The campgrounds up here are the postcard, not the bed: Kirby Cove and Bicentennial are tent-only walk-ins that don't take vans or cars, so you shoot the bridge at dusk and still drive back to Samuel P. Taylor to sleep. For the old-growth redwoods on the way, Muir Woods is the one stop worth the detour, but you have to reserve parking or the shuttle ahead and it runs 15 dollars a head.
Half Moon Bay and the San Mateo coast
For a weekend that faces the other way, run Highway 1 south to Half Moon Bay and camp right on the bluff at Francis Beach, with four miles of sand out the door. Unlike Camp Taylor, this one has hookups: most sites carry electric, ocean-front ones included, on a 40-foot cap. Book on ReserveCalifornia, and know the Half Moon Bay Pumpkin Festival fills the campground in October. That 40-foot cap leaves room for something with more beds than a van, and San Francisco RV rentals go bigger.
Fog, not heat, is what this coast does, and it can hold all day from July into September. The clear weather comes in the fall.
Yosemite and the longer weekends
Yosemite pulls the most questions from here, but it's a full day's drive each way and its own trip, so start from the Yosemite campervan guide rather than run it out of the city. Wine country up in Sonoma and the Big Sur coast past Monterey are their own bases too, and the California campervan hub has the rest of the state.