Running along the length of downtown Del Mar, without tons of easy parking, Del Mar City Beach (15th St. and Coast Blvd. to the Rivermouth, Del Mar) is touted as one of the best beaches in an area famous for its welcoming sands.
This huge strip of coastline has it all. The soft sand invites sunbathers, who can spend all day soaking up rays. The ocean beckons, the cool Pacific making a perfect counterpoint to hot San Diego [1] summer days. A decent surf break near the center of City Beach beckons to new and experienced wave riders alike. (You can even take surf lessons here.)
Lifeguards protect the shores, complete with the Baywatch-esque pale blue towers that mark all guarded San Diego county beaches. Be sure to pay attention to the colored flags on the beach. Blue means swimming is okay, and a yellow flag with a black ball means no surfing in the surrounding area.
Once you find a place on the street to park (bring quarters for the meters) getting onto the beach is easy. The area is flat and simple to access, and in some places boardwalks and paths make beach access even simpler.
Just off the beach, the Powerhouse community center offers restrooms, a restaurant, and a community theater.
Links:
[1] http://www.moon.com/destinations/california/san-diego