I think that you should to visit Ilhabela, it is a beautiful island on the north of Coast.
On the north of the state you will find stunning beaches.
Maresias, great spot for surfing & nightlife.
Ilhabela, sugest to stay 2-3 days exploring the island. You should not miss Feiticeira beach.
Perhaps you already have this in mind, but I would also recommend looking into booking a flight to Rio de Janeiro?! It's only a 1 hour flight and then you can enjoy the beautiful beaches and landscape of Rio! There are multiple flights between Rio and SP every day!
You should search to visit beaches from the north litoral from Sao Paulo state, my indications are BARRA DO UNA, JUQUEHY, Maresias and Ilha Bela - São Sebastião (I just loved the beaches in this last one, some are real paradises).