Playa Jacó offers a big, open to the public beach approximately 100 kilometers driving from San Jose. This is a great sport for surfing, takng in the sunset and if you're up for it, camping on the beach.
There are hotels nearby, but if you are staying in San Jose this is a great day trip since it is only a couple of hours away by bus. Jaco is a big, wide open beach with room for everyone. Even on packed days, it doesn't feel very crowded. It's a pretty famous surf spot in Costa Rica, but even with its popularity, you will still be likely to get your fair share of waves.
There are palm trees for good shade and to hang your hammock from. Watch out for stray beach dogs stealing your food.
