Socaltrailriders is a good place to post... in the meantime depends on where you plan to ride and what you plan to do... I can recommend Orange County and riding in El Moro/Laguna Coast Wildnerness/Serrano/Bommer (views of the ocean - miles or trail - wife can shop in Laguna beach or hang out in Newport Beach). SUggest you do some research and have a plan as to what you want to do - narrows thinsg down abit.
A shop in OC that rents good bikes is The Path. Thery have a website [thepath.com]. They also advertise on SCTR.
If you are intent of riding in LA... then there the Santa Monica Mountains or Sycamore Canyon - Malibu. Can't help you with bike rentals...
Another good website mountainbikebill.com - he has alot of ride reports, trail photos descriptions etc.
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Private docent led mountain bike rides through Limestone Canyon. Go to
letsgooutside.org and register today! Also available: hikes, equestrian rides and family events as well as trail maintenance and science study.