Jackie,
Great question!
While the Holiday decorations come down (for the most part) the night of the 25th, there are still some activities going on at the parks besides business as usual. The easiest one to point out is the nighttime entertainment on New Year's Eve. Fireworks, parades, and other entertainment in the parks are pretty spectacular on these evenings. Remember, however, that this is one of the busiest days of the year at the parks. Make sure you are in a park early that morning, and plan on staying all day, if you can. They parks usually reach maximum capacity on New Year's Eve, and if you are not in the park (especially the Magic Kingdom) by a certain time, you may not get in. They do have maximum capacity numbers that they have to follow for safety reasons.
Consider New Year's Eve dinner reservations a REQUIREMENT! Most restaurants fill up, and some apply a holiday surcharge and require a credit card to secure the reservation.
A personal dinner favorite of mine is
'Ohana. It keeps us away from the capacity crowds at the Magic Kingdom, but let's us enjoy the entertainment all the same.
Have a Magical Vacation!