>In the San Bernardino Mtns: Big Bear Lake, Lake Arrowhead, Rim of the World Hwy. A short drive east of Big Bear has you going very quickly from mountain vegetation and climate to desert vegetation and climate, even before much elevation change. Beautiful desert vistas. West from Arrowhead and you encounter similar on the way to Silverwood Lake.
>Idyllwild (San Jacinto Mtns.)Hwy. 371 through Anza to Hwy. 74, then 243 at Mountain Center to Idyllwild. Or right on 74 to go to Palm Desert and the Coachella Valley, which is why it's called Pines to Palms Hwy. Same fast change in vegetation and climate.
>Hwys. 76, 78, 79, all scenic and in some places, isolated.
>Palomar Mtn. State Park
>Pala Mission (actually an Asistencia) Pala Indian Reservation
>Missions San Luis Rey, San Juan Capistrano. And of course, San Diego de Alcala.
>Fallbrook and De Luz Road up to Temecula. Very scenic. Get off De Luz near Temecula and stay on back roads through avocado groves and get to the Santa Rosa Plateau Ecological Reserve. From there to Murrieta and I-15, or stay on back roads all the way to Hwy 74. All very scenic and surprisingly isolated. From 74 right to Lake Elsinore and I-15 or left to Capistrano and I-5.
The view from the Palm Springs Tram is stunning; you're at over 8000 ft., looking at below sea level desert. And the terrain on the mountain is gorgeous.