First off, I just want to say you are a wonderful guy to come on here and ask for ideas. Knowing that you want to make it super special for her is great! I'll share a few ideas for you.
IDEA #1- You could do the dinner then watching fireworks from the poly beach. Hire a florist to sprinkle red rose petals all over your room and have a bottle of her favorite chamagne waiting along with 2 chamagne glasses. Maybe you could get her glass engraved with "Will you marry me?" with the ring inside of it. That'd be my dream proposal but that didn't happen so I give you permission to steal it.

Plus she'll get a souvenier of how it happen!
IDEA #2- Right behind the castle in Magic Kingdom there's a wishing well and there's hardly ever any people hangin' out over there. Maybe you could give her a penny to wish upon and toss in. Right after she makes her wish and tosses the penny in the wishing well, you can get down on one knee presenting the ring and asking her to marry you. That'll be pretty romantic. It'll be like her dream came true!! To top it off maybe you can have a photopass photographer snapping away to get her reaction and everything.
IDEA #3- My fiance just gave me a great idea!! Have your GF go shopping in Japan and sneak away from her and see if the girls in the pearl booth can hide the ring in an oyster. If they can... "pick a pearl" (the oyster they hid the ring in) for her and get down on one knee and propose. That pearl booth can get crowded sometimes but if it's semi-early there's no more than 10 people there. She'll get a fiance, some bling, and a beautiful pearl all in one!
Hope this helps and good luck! We want to hear the story when you guys get back and she must start a PJ when she starts planning, ok?