We just came back from a week vacation in CR where we stayed in San Juanillo and took day trips to many of the near by beaches. San Juanillo beach was by far our favorite. It is perfect if you are traveling with children because the waves are very mild. The beach is very quiet and natural, surrounded by rocks and vegetation. There are a few paths you can safely take up the rocks and enjoy amazing views of the entire beach. Unlike many beaches in this area, San Juanillo has white sand (most others are darker or evan black sand), but there are quite a few rocks and shells so if you are not comfortable with that, I would recommend wearing water shoes. We brought them but only used them while climing the rocks.
There are also bathrooms available and a couple of places to eat. This is definitely a local beach so you won't find the usual touristy vibe, which we Loved. Few people but very friendly. There's always fishermen hanging out and boats all over. They are very happy to answer questions or you can hire someone as a guide or for a boat tour. A couple of minutes up the road there is Ancient People, a wonderful restaurant that is not to be missed.