Crescent Bay, Diamond Street, or for those with strong legs, 1000 Steps (9th Street, by the Hospital).
Crescent's a great family beach.
Diamond Street's got great rocks to explore, plus the blow hole which you have to try.
1000 Steps is isolated and uncrowded. Old School.
Avoid Aliso Creek as the Trabuco / Aliso river flows out there making the waters a bit unhealthy to swim through.