I've been wanting to visit South America for the longest time but never got the chance to. But, after finding a cheap one way deal and using miles for the return, I finally got the chance to visit South America. I was only in Sao Paulo for 1 day (on the way to Buenos Aires). I wanted to stay for longer, but that's for another trip!
If you do research online, you'll probably see lots of websites saying that it's very dangerous. While nothing happened to me when I was there by myself, I will say that you'll want to be careful. I did feel like I needed to be on guard during my visit, but any safety tips I would offer are pretty standard worldwide (not bringing too many valuables, not using your phone in a crowded sidewalk, being aware of your surroundings, etc.). I also wished that I had a Portuguese friend with me since that would had been a lot easier!
Sao Paulo is a HUGE city - even bigger than NYC in terms of area. Therefore, getting around the city can take a long time, especially during rush hour. Uber is very cheap, but if you're a brave soul and want to try using the public transportation system, it's a little confusing. Therefore, make sure you're budgeting enough time to get from A to B.
The easiest thing to do during a short-term stay is to go to a metro stop and talk to the receptionist to buy a one-way ticket. It's very cheap and all I did was hold up a finger to show "1", and that seemed to be good enough. To get the actual subway card, I think you have to be a resident. The metro is actually decently clean, but it's not the best to use it very late at night. Stick to Uber instead.
For my one day layover here, I stayed at Sleep Inn Guarulhos which did the job, nothing too special. I landed late and took a ~10min Uber from the airport to the hotel, slept the night, and then checked out and left my bag there for the day while I explored before coming back to my flight.
Overall, I'm definitely interested in coming to Brazil again (but maybe a different spot and with someone that knows Portuguese)! Highly recommend, but with slight caution if you're traveling by yourself though.
*** bolded bullet points below are ones that are highly recommended (in my opinion).
For the attractions below, I highly recommend visiting the area around them! I didn't go into the attraction itself (like there's an art museum, an aquarium, etc. that costs money) but if I had more time in Sao Paulo I would have wanted to visit!
For most of the day I was in Sao Paulo, I spent some time in Liberdade and Vila Madalena and thought they were both incredibly cute areas! Liberdade has a Chinatown-esque with a lot of Asian places infused with the Brazilian culture. Overall, there were a lot of places that caught my eye! Vial Madalena seemed to be a hip area with a lot of restaurants and bars. Lots of cute cafes around as well - I wanted to spend a little more time here just exploring.