Sorry to be a bit behind the times on this. I was in Brazil last month and I wasn't really captivated by Sao Paulo. There are things to see, but as you have identified, it's an economic (and culinary) centre, not a beauty trap. I'm in no hurry to go as a tourist.

Rio, on the other hand, I could easily have spent another week or two in. It has a bit more history (having once been the capital) and a little bit more relaxed. Certainly having a few massive (and well-known) beaches make it a popular spot with tourists and locals.

If you want to venture out a bit, the Iguacu falls are fantastic (think Niagara then multiply by 3 or 4), but there really isn't any need to go into the town. You can cross the border there - so having an Argentinian domestic flight (I think?), then a Brazilian domestic flight. Brasilia was also quite interesting and worth a day or two. Anywhere else I'm afraid I can't comment.