Sao Paulo isn't really a tourist city and the historic centre with the cathedral etc is in one of the less appealing areas. Frankly, unless you fancy braving the traffic I'd stay at the airport in one of the lounges/pay lounges/fast sleep pods or even better, get a day room at one of the hotels which operate shuttles such as the Marriott & Sofitel Pullman, or the cheaper Panamby Guarulhos or Mercure Guarulhos a bit further away. BA & LATAM use the new Terminal 3 which is a much more pleasant place to stopover than the other two.