Sometimes, SDAR (San Diego Adventure Riders) people hit Idyllwild, Big Bear area. I rode at Big Bear just last weekend.
Yes, Big Bear is a great DS friendly area. Great Trails on the South Slope in the Angeles Oaks area, Holcomb Valley and Fawskin areas. Go down Hwy 18 to Cactus Flats which leads to Pioneer Town/Yucca Valley or Johnson Valley.
I live in the Lake Mathews area and ride all over rierside county. I can ride from my house all the way to the factory supercross tracks without hitting the pavement. Same to Lake Elsinore. Perris mx track is about 10 minutes down the road and I ride there quite a bit as well as milestone.