Cab to/from GRU would set you back around BRL 120 each way, which is why I suggested renting a car (prob BRL 100 including insurance), but your concerns about driving around are valid: hard to navigate.

I think your best bet is probably to get a bus to a hotel, as suggested, and cab around.

Have you tried hitting the forums on Tripadvisor? There could be referrals of good guides who could set up interesting itineraries for you.

Another option, with private guide picking up/dropping off in GRU (maybe negotiable?):
http://www.toursbylocals.com/SaoPaulo