You can pretty easily ride from DTLA to Big Tujunga Canyon, which leads (eventually...) up to the open part of the Angeles crest. Even if you don't go all the way, it's a pretty nice ride once you're on Big T. You can also take a bike on the gold line to get to Pasadena or Sierra Madre and ride from there up the 39 or GMR (two of the nicest rides in the area).