That is a fantastic place for riding. Go north. My preferred route would be 56 bike path to Camino del Sur, head north, hang a left on San Dieguito, then right at El Apajo. Just ride through the streets of Rancho Santa Fe - gorgeous homes, minimal traffic. If you want a longer ride (probably 50 miles) head east on Del Dios (it turns into Valley) then loop through Harmony Grove/Elfin Forest. All of these streets either have bike lanes or tend to be frequented by lots of cyclists.

or west on the 56 bike path to the coast, then up 101 to downtown Carlsbad and back

Agree with jwizzle about the 56 bike path to CDS, then north. However, I would turn right on Carmel Valley, several nice hills to Camino Del Norte, left on Camino Del Norte which turns into Camino Del Sur, right on San Dieguito, past El Apajo to El Camino Real, right on El Camino Real, left on Via de la Valle and then a right or left onto PCH. I like driving El Apajo, but it's not for the faint of heart from a bicycle safety standpoint (IMO). Too many curves, skinnier than normal roads and no bike lane. Have fun.

My recommendation would be to take the bike path out to the coast. If you're coming to San Diego, ride along the ocean!

You can also hit up Torrey Pines (up the inside, down the outside). I'd say ride north along the coast first, maybe turn around just before Carlsbad, then throw in a few Torrey repeats at the end before heading black inland.

Just note that you actually have to take Carmel Valley Rd. from El Camino Real, so really just a short distance on the bike path and then you have to navigate a few sketchy intersections by the fwy interchange.