Chile has plenty of coastal towns that are popular destinations for summer beachgoers and surfers; one of the more famous ones, Pichilemu, was actually built to be a resort. In the late 1800's a wealthy Chilean politician, Augustin Ross Edwards, developed the area into a resort town that included one of Chile's first casinos. The casino is now a cultural center, but the town is still a well-known tourist destination.
Pichilemu is about 200 km southwest of Santiago. If you're going there by bus, take the line from San Fernando; buses depart hourly, and the trip takes about 2.5 hours. The town itself has colectivos, which have routes through the town and also to Punta de Lobos, about 7 km away from the center of town.
If you're interested in surfing (or learning how to surf), there are plenty of shops in Pichilemu where you can rent surfing equipment or hire an instructor. Punta de Lobos is a great place to relax or enjoy the waves, with food stands and a few restaurants, and an excellent view of the Pichilemu coastline. For local flavors and fresh produce, go to the open market (open every Saturday) on Calle Ramon Freire. You can also take a colectivo to Cahuil, a scenic town about 14 km away that's mainly centered on salt production, or inquire in Pichilemu for a "salt tour" if you'd like something more educational.
The main tourist season is between mid-December and the end of April, and the cost of hotels or hostels can be double what they normally are during this time; most other prices, such as in restaurants or taxi/colectivo fares, are also much higher. If you visit anytime from September to early December, you can still get good weather, but you'll also have smaller crowds and lower prices. Regardless of when you go, you can still enjoy the coast and the surf, have some good food, and make new friends.