You can get inexpensive chargers that hold a couple of protected 18650 Lithium ion cells. You charge the cells on AC, then the charger can use the cells to charge other items via a USB port, or the cells can be used in a bike headlight.
Silverwood Lake State Recreation Area, about 35 miles from San Bernadino at 3000 feet elevation, has some nice hike and bike sites. The store in the campground had a limited selection of food when I stayed there a few years ago, so bring your own gourmet selections if you try a test overnight there. Also be prepared for some short steep sections on the winding road between I-15 and the lake.