If I am not mistaken, Ilha Grande can be a quick day trip from Rio. And I suspect that newyorkgeorge will confirm that you will barely have scratched the surface (of beach-going, that is! ) after spending 2 days at the Farme beach in Ipanema in Rio. With any luck, the rains we are now having will have moved on, and the Rio beaches will have reverted to their normal decadent selves.