Point the van up Highway 74 out of Palm Desert and drive the Palms to Pines switchbacks while the Coachella Valley floor drops away behind you and the desert palms turn to pinyon and then ponderosa. The night is at the top, at Idyllwild Campground inside Mount San Jacinto State Park, a short walk from town under San Jacinto's granite. Book that site on ReserveCalifornia first and the van second. Summer weekends up in the pines go early, and a van with nowhere to park it is a long drive back down the mountain in the dark.
Idyllwild takes reservations all year. Stone Creek, the park's other drive-in up Highway 243, is the place to look when Idyllwild is full and the season is warm enough for it to be open. If you'd rather have the room of a full RV for the same loop, the California listings cover the state.
There is a second way onto the mountain that skips the drive. The Palm Springs Aerial Tramway lifts you out of Chino Canyon into pine forest in about ten minutes, with the valley laid out below the cars on the way up. Call it a half day. Everything above the mountain station is hike-in wilderness, so the van waits in the lot and the night still happens somewhere else.
Indian Canyons
For a morning without leaving town, walk the Palm Canyon trail at Indian Canyons off South Palm Canyon Drive, where a creek runs through a grove of California fan palms at the foot of the San Jacinto wall. The tribe's separate Tahquitz Canyon unit trades the palms for a stone-stepped climb to a waterfall bowl. Both are day-use, and from early July into fall the gates open Friday through Sunday only (indian-canyons.com), so go early, before the heat, and sleep somewhere else.
Joshua Tree and Anza-Borrego
Joshua Tree National Park is close enough to run in for the day from a valley base, and its campgrounds, timed entry, and van notes have their own page at Joshua Tree campervans. South of the valley, Anza-Borrego Desert State Park is the other big desert drive, covered from the San Diego page.