The cake and the decorations will be in the room when you arrive. They have silverware in your room to go with the cake, but for an added touch, I brought some Disney paper plates and napkins.
Here's the decorations and cake (the first pic is a magnet)
http://i302.photobucket.com/albums/nn102/***********/DisneyDream2011086.jpg
http://i302.photobucket.com/albums/nn102/***********/DisneyDream2011085.jpg
http://i302.photobucket.com/albums/nn102/***********/DisneyDream2011084.jpg
http://i302.photobucket.com/albums/nn102/***********/DisneyDream2011083.jpg
http://i302.photobucket.com/albums/nn102/***********/DisneyDream2011073.jpg
Another birthday idea is this: If you get a chance to go into the gift shop without DD with you, you can go purchase something for a B-day gift, then take it to guest services and tell them you want it delivered on her actual birthday at dinner. This is what I got out of the gift shop (I bought the B-day pin separately and attached it to Minnie's hand)
http://i302.photobucket.com/albums/nn102/***********/DisneyDream2011720.jpg
Also, go to guest services and ask for a birthday button (similar to the ones at WDW)
Another thing you can do, is drop off a photo matte, pillowcase, or something to get signed by the characters. They won't deliver it till the last night of the cruise though. On our meet thread I was talking with someoine and we were trying to figure out if they'd sign a greeting card. Well, they signed one for DD. So here's what I got signed (on the photo matte, I added the stickers after I recieved the matte):
http://i302.photobucket.com/albums/nn102/***********/signaturesDCL002.jpg
http://i302.photobucket.com/albums/nn102/***********/signaturesDCL003.jpg
http://i302.photobucket.com/albums/nn102/***********/signaturesDCL004.jpg
Hope that gives you some ideas. I tried to do as much as possible for DD cause it was her 16th birthday!!