I was at MGM for new year's eve to ring in the 2005 year. At MGM they have an awesome fireworks show (which I think they do at every park) and I think they had a band play before the fireworks. They set up adult beverage carts and there are areas where you can go and get a New Years top hat for free. (It's black and plastic but still cool).
Here is an important tip, every park will fill to capacity and early and each park will be packed all day long. Despite the huge crowds, I had a ton of fun, hope you do too!