I have a bunch of things to decorate rooms in my 4shared, look in the "Mickey's Grand Welcome" file. There are banners, balloons, footprints, and things for Mickey and the Gang, Tinkerbell and the Princesses. You could decorate her room while she sleeps and then she could wake up to the "disney" surprise. Or you could decorate it while she is at school so she could come home to the surprise.
On her bed you could leave a present wrapped up in bright colors or disney wrapping paper and a big bow with Mel's countdown chain and a mylar disney balloon inside. Then tie a letter to the end of the string on the balloon to let her know she is going to Disney. Then it would be just like the commercials where the disney balloons float out of boxes.
You can also find Minnie things to decorate her room in the "parties" file and "Minnie" subfile.