Sao Paulo really needs to be seen (from the air / land) to be believed - it's absolutely massive, and feels far bigger than any other city I've ever visited (including most major North American / Australian / European cities). Don't underestimate the traffic, which makes London look easy! Don't recall there being good public transport (as opposed to taxi) links when I was last there in March 16.

The Marriott GRU is good for a quick overnight stopover although with just 8 hours I'm not sure it would be worth the hassle.

From your signature I think you have OneWorld Emerald status, in which case the LATAM lounge in GRU is huge, comfortable and includes (among other things) a sleeping area which you may find useful, although you probably have a daytime stop IIRC from my last trip to GRU. The AA lounge is literally next door, which is also worth a look although isn't quite as good in my opinion.

Have a fun trip