What you would like to do at the beach? Just enjoy the beach and the sea and, when you get hungry, eat some seafood!
The beach is not serviced not guarded, you'll have to accommodate yourself and help yourself out.
If you would like to do some watersport, your best bet is to check the only kind-of-resort on the beach, the Los Cóbanos Village Lodge, that offers to their guests - but I think to whomever in interested - snorkeling, diving and other water activities such as surfing, kayaking and windsurfing. The Lodge is located at about half the beach; pay attention as you walk by since the entrance is not easy to spot.
There are few eateries on the beachfront for those who get hungry, however only one was open the day I visited.
The beach at Los Cóbanos is easily reached from Sonsonate by a ~45-minute bus ride. At the bus station in Sonsonate check out buses departing to Los Cóbanos as Los Cóbanos is the final destination of that line. I am not sure there is a timetable, just get to the platform where they leave and wait for the next one departing.
In Los Cóbanos, the bus will drop you in the middle of the village (that the average tourist will have hard time in recognizing as a village), at a short walk from the beach. As you leave the bus, you may be approached by individuals offering you meals and rooms to stay. On the way back, buses depart a little further down the road from where you came, after the first curve.
There are no taxis in Los Cóbanos. If you are not into taking the bus, you shall have your own transportation.
The village of Los Cóbanos, and the whole area around the beach, is a poor area and you will find a number of loosen animals hanging around. Besides the few stray dogs, quite usual in the whole El Salvador, even a loosen pig crossed my path! Be careful.