Mark,
Great plan already! You get a vacation out of a proposal. Most guys just do it over dinner, so you are already ahead of the game!
So there are naturally a ton of places to go, as there are Photopass photographers literally everywhere. But you need to consider what the crowds in the park might do to the magic moment.
For example, many try to do this right on Main Street, or in the hub in front of Cinderella Castle. But that is where crowds are always plentiful. And sometimes people are, to be generous, a bit oblivious to what is going on around them. So your proposal picture might have Lou from Des Moines screaming at his kids to hurry up. Definitely NOT magical.
My advice is to use the castle as the backdrop, but position yourself better. On the bridge that leads from the hub towards the Mad Tea Party there is almost always a Photopass photographer, the crowds are lighter and more focused on going in a single path that should not get in your way, and you can get against the northernmost railing for the shot, reducing the chance of a accidental intruder wandering into your picture.
No you can go one step further, and work with Jean Volante's team at
Disney Floral and Gifts Dream Makers. They can transform the moment into art-quality photography that might make for an even greater story.
Read here to learn about what they can do!
Write me back if I can help with more suggestions, based on specific locations that are special to you!
Have a magical vacation!