In Rio you should heed [MENTION=793168]Esmjb[/MENTION]'s advice, Those are the best locations, unless you want to splurge (US 170-200) and stay at the Sofitel which is the best location in Rio bar none. The Copacabana Palace would be memorable, but the location is not so good (but still a once in a lifetime experience worth doing if you find a good rate).

In São Paulo, especially during weekends Accor is your friend. Their rates are excellent value, and I often get the newly refurbed Grand Mercure Vila Olimpia (previously Caesar Business) for an incredible U$35 per night! The attached Pullman is also nice but not such good value. Mercure Funchal is also nearby. All these options would be easy to get into CGH on Monday. This area has high-end shopping malls (JK has some nice outdoor terraces with good cityscapes) and restaurants in nearby Itaim. Another option is to stay in Jardins, in which case you'll find the Mercure Jardins to have a great location to walk around Av Paulista and explore the points of interest downtown (and Liberdade).

In SP on a Sunday, you can/must go to Ibirapuera Park, and check out the view from the Modern Art Museum (MAM-SP) across the road. Walk around Vila Madalena, and visit Beco do Batman, with graffittis. Check out Liberdade, the japanese neighborhood and grab some good sushi or noodles.

It's easy to get around on a Sunday with no traffic and Uber.

Iguazu: do not stay (or even go) downtown: it's uninteresting and a waste of time (and resist the temptation to get an extra stamp on your passport and go to Paraguay, which is a real dump). Consider staying on the argentinean side which is the more interesting side of the park (half day on the Brazilian side of the falls is more than enough), and has better F&B options with argentinean steak & wines. There are lots of "jungle-style" lodges in Argentina, or B&Bs downtown.

Salvador: the Wish Hotel (formerly the Sheraton) is great value and is close to the historic center. The Mercure in Rio Vermelho (or the nearby Pestana and Golden Tulip) is close to some nice restaurants for dinner (Casa de Tereza is a must). If you're there on a Saturday, sunset jazz at the Modern Art Museum at Solar do Unhão is a must. I wouldn't stay in the historic center as it gets a bit sketchy at night, and the constant hawkers wear you down.