The problem with São Paulo is that everybody has the same idea, and it is a huge city with lots of cars. Everybody wants to head out of the city to the beach on a nice weekend. The traffic jams are infamous in the summer. People also head to the Mantiqueira mountains for cooler temps.
You could head for Santos or Guaruja, probably the closest beach towns.
Ilhabela is a bit further up the coast, about 4 hours (200km).
Paraty, with a Colonial center and nearby Trindade beach, is about six hours.
Or maybe south of the city to Peruibe.
Embu is a small town known for its crafts shops.
You could go to the mountains, Campos de Jordao for example (about 2 1/2 hours). The area is very popular during the 'winter'.
Or just take a shuttle flight to Rio.