To be honest with you I'd skip trying to do Rio and Amazon in one short trip. As a previous poster suggested, there is absolutely loads to do in and around Rio and certainly enough to keep you occupied and give you a taste of Brazilian life.

If you do choose to go to Amazon, keep in mind that it is around a 5 hour flight from Rio, so both the flight there and the flight back will effectively knock the best part of a day from your time in Brazil - after taking into account the time it takes to get to the airport, hanging around at the airport, flight and then luggage collection and taxi at the other end etc.