Here's what I'm going to do but you can steal it if you want it. :lol: It's kind of corny but some girls eat that stuff up.
Take her to Cinderella's Wishing Well. Give her a coin to toss in the well and tell her to make a wish. Tell her not to tell you her wish because then it won't come true. Then tell her it's your turn. Pull out a coin and toss it in. She'll be wondering what your wish is even if she doesn't ask so say "Let's see if it will come true." Then get down on one knee and pop the question.
Kind of corny, but I thought it was sweet. I'm faaaaar away from getting engaged so feel free to steal it.