If You Have 1 Day -- Get to know South America's largest city. Wander the busy pedestrian streets of the old downtown neighborhood of Centro. In the constant commercial chatter you'll feel, see, and hear Paulistas at their best: buying, selling, and trading. Ascend to the top of the Banespa building for a 360-degree view of the city. Have a coffee at the Patio do Colegio. In the afternoon take the Metrô out to the Avenida Paulista. Bask in the wealth and power a bit, then go see some fine art at the MASP. In the evening take advantage of your presence in Brazil's culinary capital and go out for dinner at a truly fine restaurant like the Antiquarius or Figueira Rubaiyat.
If You Have 2 Days -- Get some culture. Anyone interested in modern Brazilian architecture should see the Monument to Latin America. Those interested in seeing Brazilian art should check out the Pinacoteca do Estado, or for older work the Museu de Arte Sacra. In the late afternoon or evening, reward yourself with a stroll through the Japanese neighborhood of Liberdade, topped off with a fine Japanese supper.
If You Have 3 Days -- Take a break and spend the morning shopping in the green and leafy Jardins neighborhood. The intersection of Rua Augusta and Oscar Freire is perhaps the most exclusive shopping enclave in all Brazil. Ditch your purchases in your hotel, then go relax in the afternoon in Ibirapuera Park. Stroll the pathways, rent a bicycle, and enjoy the people-watching. That night, check out some of the clubs or bars in Vila Olímpia, starting the evening with a chopp (draft beer) and some picanha at Rabo do Peixe.
If You Have 4 Days or More -- Enough! You've earned a break. Head down to the coast, and enjoy the largely untouched paradise that is Ilhabela.