Catch some waves on Playa Zicatela - if you don't surf, you should at least make it over to the beach restaurant Los Tios and have a beer on the beach while you watch people surfing in the sunset. Also a good place from which to watch over breakfast when there's a surf competition on.
Playa Carrizalillo is a good beach for swimming or learning surfing with few restaurants.
The Andador is the path around the lighthouse. It is a fun walk, and beautiful, but the surf can be dangerous in spots.
The best and safest beach for swimming is Playa Carizalillo. Take a taxi unless you want a half-hour walk. Carizalillo is small, gorgeous, and not too crowded, 167 steps from street level down to the beach then back up again. There are some surfers and some boats, but mainly swimmers enjoying the waves.