It's been years since I've been to Sao Paulo so I don't remember. I've done some poking around online and can't find an answer for you about lockers. However, what about a cheap airport hotel? This would give you the chance to freshen up, nap and leave your luggage.

Sao Paulo has a couple of good museums. Check out www.Frommers.com and search for Sao Paulo to find them. From what I remember traffic in the city if horrific, but then what do you expect from a city of 17 million. Depending on the time of day it could take as much as a hour to get downtown. You could take a taxi or there are express busses from the airport.

If you're feeling really adventureous you could rent a car and drive to Santos. It's a coastal port city a couple of hours (or so) southeast of Sao Paulo. Not as nice a Rio or Recife though. It's also the home to the legendary futebol player Pele. He has a massive compound in the city. Seafood can be particularly good here too. But probably the best part of going to Santos is he drive itself - breathtaking, mountainous, tropical jungle.