Hands down, Topanga Canyon. It won't be hot. Drive N on Lincoln to I-10W, becomes NB SR-1 (PCH), turn right on SR-27 (Topanga Cyn Rd). Stop in town of Topanga, pick up picnic supplies/beer/tofu at health food store or grocery. Continue maybe 3 mi to Topanga State Park.

Also Tapia State Park, Malibu Cyn Rd N of Malibu, features hard-to-find swimming holes and famililar scenery (you'll recognize the skyline instantly as the opening scene from a well-known TV series).