An absolutely astounding beach favored by surfers due to high waves, Boiçucanga is about 90 km out of Guarujá proper. It's worth the drive, however, (you can get a bus at the Rodovaria Guarujá or directly from São Paulo) even just for the view along the way. The tropical forest is a lush vegetative sight to be seen, especially where it seems to war with the water. Although generally less crowded than the beaches in Guarujá proper, it's to be avoided during the Brazilian holidays - especially those that span a long weekend. Paulistas with a little time and a little money will be flooding the area.
We stayed at the Pousada da Barra (012 - 465-1391; R$80-100), and it was a nice place, with a sweet gentleman that even spoke moderate English. Frankly though, I'd be tempted next time I go to follow up on the campsite that I heard was in the area. For those backpacking, I hear there's a youth hostel too.