In Praia Grande you will find a long beach (the translation for Praia Grande is Long Beach). The beach has fine white sand, and moderately big waves, which are great for practicing beach sports, surfing mainly.
The ride from São Paulo to Praia Grande will take you around a couple of hours. You can go there and back the same day. The first part of the ride is spectacular. The road swerves through the region's green mountains, on huge bridges and through tunnels. The infrastructure is great, but the views from the bus are even better. When leaving São Paulo you will get to see some of the favela neighborhoods too.
Bars and restaurants on the beach are cheap. Do not miss the opportunity to have lunch on the sand.