It´s a tough life in a beach town, especially in summer when the time comes to choose what beach to go laze the day away. Playa (beach) Miraflores and Las Salinas are right on the doorstep to most major resorts and are the ones near the Muelle Vegara (the big crane overseeing the ocean)

Las Salinas Beach (Photo Emilio Fuenzalida)
Incidentally, Muelle Vergara is earmarked for demolition as of this date because of unsteady foundations and no buyer to come and resue it, so the beach will soon be jetty free.
Going further north is the popular Reñaca beach. Reñaca gets a bit crowded, but has a special section called ¨el cementerio¨ the cemetry oddly enough, where all the beautiful people like to be seen. We recommend having a Chilean Gelato while your at it.
Don´t forget to take an umbrella and sunscreen because the Chilean sun doesn´t tickle if you get burnt.
More info for spanish speakers
here