Those are all good suggestions but you can beat the heat and stay closer to LA with these too. I usually do just an overnighter or two at any of these locations.
Caveat: Craig Wisner can actually elaborate on a couple of these that he's just recently introduced me to.
Angeles Crest: Horse Flats 5000ft'ish, good local rocking climbing as well. An hour or less from Pasadena.
San Gabriel River Valley (Azusa). Craig just brought me in here. I can't recall the trail head name. If you plan to hike a mere 4-5mi in you'll leave the locals and prospectors behind (yes, I said prospectors).
Los Padres NF:
Sesepe Mountain Range. 1.75hrs from LA up the 5. Exit Frazier Park and head West. A river valley. Hot Springs. Plenty of wet camps (even in Aug or Sept).
Anyway, look me up next time yer around. I'm sleeping out most any weekend, and always willing to meet a fellow BPL'er.