First off, congrats! What a sweet idea to propose at Magic Kingdom, you will have great memories and photos!
I don't have a suggestion about how to tie the donald theme in. I know many people have tried to get characters involved somehow and its complicated and many times not possible. Sometimes the more simple it is the less stress you have. Here's a very simple but romantic idea that my friend recently did for his girlfriend's proposal...
He had the ring in his pocket and when they got to Magic Kingdom, he said the first thing he wanted to do was go to Space Mountain (because of the walking path they would have to take...). On the walk to Space Mountain, there is a bridge that is to the right of the castle. There is a little secluded photo area there (before you get to the bridge on the left) in the rose garden where a photo pass cast member always stands. They got into photo position and as the photographer was snapping the first photo, he sank down on one knee and proposed. The photo pass cast member saw what was happening and immediately started snapping dozens of great shots as he put her ring on, she cried, they hugged & kissed.
The backdrop of this photo area is gorgeous, with the roses and the castle in the background. And the photo CM are great about capturing the "moment" and not rushing you. But there are many other photo pass areas around...maybe just carry the ring in your pocket until you see the right setting...and then do it! All the photo pass CM are amazing at their job and will know what's going on the moment you get on your knee. If your nervous about them getting the "right shot" you could somehow whisper to them what you are about to do when you walk up to get the photos done.
This way you get amazing professional photos of the proposal - which she will cherish. You can have them printed at the park, or the photo pass people will give you a little card that you can go home and order prints online, or even better, purchase a CD of the digital photos.
Good luck and most important, have fun!
