I lived in Brazil for a year from 2003-2004 and I traveled back to Brazil for five weeks in 2007. I have seen a lot of the country, but I still have a lot more to see. Here are my recommendations of the MUST SEE spots in Brazil:
Iguacu Falls: Cute animals. Beautiful waterfalls. Easy to get to Paraguay for a day of shopping.
Sao Paulo: Huge city. Tons to do. Recommend exploring for at least a day!
Santos, Sao Paulo: Beautiful beach town with few non-Brazilian tourists (since most everyone goes up to Rio). Highly recommend.
Northeast Brazil: If you love beaches and culture then you must head to the northeast of Brazil. I especially love the state of Bahia. Amazing culture, beaches, and food. MUST MUST MUST!
Rio: MUST SEE. Obviously go to Sugar Loaf Mountain. Hang out on the beaches. Just explore! Amazing city!
Florianopolis, Santa Catarina: Another amazing, beautiful city that does not get a lot of non-Brazilian tourists. There is an island and main land and so much to do including eating seafood, sand boarding, and tons more.
Curitiba, Parana: I love this city. Not a huge tourist destination, but nice to visit if you have time. Can get really cold in winter.
Ouro Preto, Minas Gerais: Must see if you like history and/or historical towns! Lots of culture here!