Puerto Escondido is a town in Oaxaca known for its big waves and excellent surfing conditions.
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.
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. A great place to spend the day drinking coconuts cut from the tree, chilled and filled with alcohol.
Visit Puerto Escondido before its turned into another Cancun! When I was there last, there was a lot of construction going on.