Hello,
That is such a wonderful Idea! My wife and I went there on our honeymoon and it was great.
I h ave a couple Ideas for you. However, the big question is do you want a lot of people around. It may be hard to do it in a secluded place there, but anyway here is just a couple thoughts.
1. On the cinderella castle grounds there is a wisheing well, it is off to the right as you are walking to the castle from the park enterance. Not usually too many people around, but great views of the castle and the grounds.
2. Perhaps in Italy at epcot there is an a beautiful fountain there that could be very romantic!
3. Perhaps this one may sound odd, but right in front of cinderellas castel is the stature with walt and mickey. It just says it all for Disney lovers, what a great place to start your life together.
4. This would be my choice, but the problem is always a lot of people. It is During the fantasy in the sky fireworks at magic kingdom Perhaps you could find a place be a tree or something with the lighted castle it the background.
Well, good luck post more if you want any more info