from the top of Mt Lowe you will see a few "lines" over on San Gabriel Peak's south face, one obviously near the left edge of the cliff; what looks like a system of corners and steep chutes
I've scrambled up to it and climbed up a ways and the rock seemed like decent quality (though of course I got hit by a falling rock--very typical for this area) ...but it did not seem like it was worth drilling a few bolts and dealing with the assorted choss on a route that just finally led to steep slopes covered with manzanita and chaparral...but who knows?
the "Super Couloir" on the south face there is a constant shooting gallery of falling rocks...just past the tunnel there I got hit by another stupid rock, just eyeing the initial 35ft of hard-to-protect 5.9ish entry moves. I'm sure someone's done it.
Horse Flats and its Top Rope Area are the real ticket, after the frogs finally took over at Williamson.
if you are inventive, there's also bouldering, and traversing on small cliffs in the Arroyo Seco, Winter Creek, Upper Millard and Big Santa Anita canyons, prolly even some top-roping there.
In Rubio Canyon, bushwhack way past Thalehaha Falls, and there's another giant waterfall up there, prolly some climbing beside it.
Crystal Lake crag seems like great granite for the front range, with a few quality leads. Alder Creek and the San Gabriel River used to be popular, I think, for top-roping and leading.
the cliff on Strawberry Peak has provided people with some wild adventures, but be prepared for falling rock and big loose blocks
wandering up Sunset Rd beside upper Millard Canyon you will spy some big cliffs across the ways there...they look promising but will involve a major bushwhack/scramble...most people just drive to Josh!
here's another chossy adventure in the front range--I've done it ropeless a few times...kinda scary!