If you're willing to go to Carlsbad, Hwy 101 from the north edge of Leucadia to the Oceanside Pier is pretty good. At Oceanside, get off of 101 as soon as you enter the city and get over to Pacific Ave. If you feel like a longer ride, go over to the San Luis Rey trail, and take a nice ride inland for about 7 miles. Great for biking, but make sure you go to the bathroom before, because there's nowhere but the bushes on the trail.
If you really want a nice ride, drive up to Pulgas Road and park in the lot by the start of abandoned Hwy 101. You can take that all the way to San Clemente -- a really nice ride. If you're adventurous, you can follow the bike route thru San Clemente and there's a great trail all the way to Dana Point.
I know -- that's kind of out of San Diego -- but you can start it all in Carlsbad!