You don’t give us much to go on. Some people would happily spend the days sitting on the beach in Rio and the nights clubbing. Others will want a new city every few days; then there are the ones who want the Amazon or black culture or music or … the list could be long.
So Definitely go to Rio for at least three days and do all the standard touristy things.
The Foz do Iguaçu https://en.wikipedia.org/wiki/Foz_do_Iguaçu have one of the most spectacular waterfalls in the world. Eat your hearts out Niagara and Victoria Falls. You can also walk over the bridge into Paraguay and see Ciudad del del Este. This is the nearest you will get to the Wild West but with shopping instead of cows. https://www.tripadvisor.com/ShowUserReviews-g298437-r20742517-Ciudad_Del_Este_Alto_Parana_Department.html The next day take the 20 minute bus journey to Argentina and see the Iguazu Falls from the other side of the river. This is by far the best side. Stay a day or two and make sure that you eat a parilla (barbecue) and a Bife de Chorizo (a fabulous steak.)
Salvador is a very interesting city with a lot of history and black culture. It is hard to miss Olodum, their drummers are all over the city. Try to coincide your visit with a festival where the massed bands of drummers play.
December is the height of summer and it will be VERY HOT!! If you want something cooler go south, especially to the mountains. Campos do Jordão in São Paulo state is well equipped for tourism and is less hot.
If you are into serious food (and can afford it) São Paulo is the city for you.
Tips: Take plenty of sun block, don’t even think about driving a car and be very careful, especially at night and in deserted places. Take taxis at night - even for short journies.