Where are you seeing rain in the forecast? I've been stalking the weather for our NYE trip, and on the 2 week forecast it shows 0% chance of rain starting Dec 23 going until the 29, and in the 70's every day.
For rain tips, I've got plenty. We were there on a miserably rainy day last year. We went for a day trip for DD's 5th birthday.
#1- Get ponchos before you go. First, we bought the cheaper Disney Ponchos. DD's ripped just putting it on the first time, so we exchanged it. Both DD's and DH's ripped several times throughout the day, and we kept having to exchange. Finally we upgraded DD's to the more expensive Minnie poncho, but that one is not very long, didn't cover her legs at all, so she got soaked. But at least it didn't rip. Then, Disney eventually ran out of ALL ponchos, I couldn't believe it. So for SURE, buy before you go.
#2- Parades, Fireworks, Fantasmic were all cancelled that day.
#3- Hit up the Tiki room show, Innoventions, and all the indoor stuff. I actually appreciated the rain a little because it forced us to see some of the things we never would've paid much attention to before.