Unfortunately, anything and everything.
I'm 3 hours north of WDW. Yesterday, it was sunny with highs in the upper 70s. Sunday night we had a "hard freeze"...temps below feezing for at least 8 hours; they were in the 20s.
Today, highs are expected to be up in the 70s again, but with rain. When the rain is over, we'll have a couple of nights with temps around or below freezing, with the expected high on Christmas day to be around 50.
Orlando is about 10 degrees warmer this time of year. So you'll have a couple of warm, shorts wearing days followed by chilly, better have a jacket days, followed by more warm days, with more chilly days. There will also be some rain, but probably not the torrential summer shower type.
The weather this time of year is VERY dependent on cold fronts, which are notoriously difficult to predict. They frequently stall, so a day will be warmer or rain longer than predicted, or they come in faster than anticipated, making a warm day suddenly turn cold and windy.
There are many links to weather forecasts in this forum. Use those as a guideline, but be prepared to layer, especially if you plan on watching the late night fireworks, shows, etc. All of the CMs here can tell you that the shops do a booming business selling sweatshirts, blankets, etc. to tourists who assumed it would be warm and sunny the whole trip.