Depends on where you stay. If you base yourself out of, say, Carlsbad, you're in pretty good territory. PCH for longer, flatter rides, Torrey Pines climb if you do a 3 hour (to me) round trip ride.

Oceanside and Camp Pendleton north. Celo Velo or some team by that name does rides from Oceanside. SDBC seems to have rides south of Carlsbad. Swamis if you're a Cat 2 or better.

Palomar would be a 100 mile day, out and back, with a couple other climbs along the way.

Lots of bike lanes etc.

cdr