Rock Climbing is a popular sport as Joshua Tree National Park. The stunning rock structure provide ample challenges in climbs that offer perfect weather (though summers can be really hot & dry) and heights that make the climbs do-able.
For that reason, there's a rock climbing season in which athletes can sign up and take lessons. Located around 2 hours outside Los Angeles in the Palm Desert, you can make it an easy day trip from most parts of Southern California.