Firstly, have you booked accommodation for Rio New Years? You will be lucky to find a room at this stage, and if you do, the prices will be high, and they always make you book for x number of days (think it's 4 or 5) Totally not wanting to put a dampener on this, just want to make sure you're not expecting to turn up and get something in Rio at that time of year. If you want a contact for apartments if you're struggling, then let me know as I have a couple of people who might be able to help (I lived there for 5 months)
Ilha Grande is the same either side of New Years with people en-route / leaving Rio - you will have to book.
I was in Rio for New Year last year, and I headed to Buzios after New Year - I'd never been before when I lived in Rio, but it was beautiful. The only 'problem' was it's rainy season, so it rained the whole time I was there - but that can happen anywhere really at that time of year, it is just your luck (It's boiling hot at the other extreme when it's not raining!). Buzios might be a good place for Christmas - it's lovely, and lots of nice beaches too, then you're just a few hours north of Rio to get there for New Year...but as Waking Dream said, it'd be good to know where you're likely to be just before Christmas, then we might be able to make better suggestions
I'm so jealous!! Make sure you get your white outfit ready for the Copacabana beach party at New Year!!!