My DH proposed in December at the end of Mickey's Very Merry Christmas Party. During the later parade, he bought me one of those red light-up roses. After the parade he asked me if he could "look" at it. As we were walking to Space Mountain, he stopped me on the bridge to the right of the castle. This was the bridge near Cosmic Ray's - close to castle. He handed me back the rose with the ring on one of the leaves!

It was kinda cheesy, but very sweet and romantic! We always stop at "our spot" on that bridge every trip.
We got married at WDW about 11 months later. The ceremony was on the beach/grass at the GF (now the pool

) and our reception was at the California Grill.
I love all the ideas you have here. And it seems like you have a pretty decent budget too! Since I am not such a public person, I like the ideas of the cruise, dinner at the castle & proposing after in front of castle, etc. The private couples massage with private dining sounds wonderful too! You could also do crazy things: a friend of mine got engaged while they were riding Splash Mountain! You could propose on a monorail ride in the front cab. Maybe you could send her to a day of beauty at one of the spas - while you set-up the big proposal?!
I'd call Disney's Fairytale Weddings - I know there is a dept. that specializes in setting up proposals. I bet they have alot more ideas than the ones you are already getting here.
No matter what you end up doing, it will be perfect for you two. CONGRATS!