There are a lot of great rides in the San Gabriel mountains, San Bernardino mountains, Santa Monica mountains, etc.. From our house about 15 miles east of downtown L.A., we can take the San Gabriel River trail up to Hwy 39 up into the mountains past Crystal Lake to Hwy 2 and go all the way to Wrightwood, 65 miles each way, and reaching nearly 8,000 feet, with only one traffic light for the whole trip. Mt. Baldy and the ski area at 6500' IIRC are popular destinations for cyclists' day rides. We normally go up there by way of Glendora Mtn. Rd. and Glendora Ridge Rd. in order to avoid more miles of city streets which we get when we go up Mt. Baldy Rd..
We tried to ride to Big Bear Aug 4th but didn't finish the ride because our younger son got in a traffic accident and broke his collar bone at an intersection in Rimforest after we had completed most of the climbing. I would not recommend riding up Hwy 18 in the summer when there is a lot of traffic with RVs (and renters driving them!), fifth-wheelers, very wide boat trailers, etc. on the very narrow last half. Definitely use a mirror. I had to motion many times for people coming up behind to leave me more room or just not pass until it was safer. They complied; but I might have been knocked down over a cliff if I didn't have the mirror.
Most of the California coast makes for excellent riding as well, but most of the year you will want to ride from north to south, not vice-versa, because of the wind. Take Amtrak up to your starting point and ride down, or, if you just ride to San Diego, ride down and take the train back. From the Santa Ana train station to the San Diego train station is about 100 miles. From the Fullerton station it's about 107 if you ride the freeway through the Camp Pendelton area, maybe a little more otherwise.
I would like to learn the route out the Santa Ana River trail to Riverside since I have a couple of sibblings out there. It may not be the shortest route for me, but using the trail will cut out a lot of stop-and-go city riding.