My first advice would have been to have a plan that doesn't involve asking NS what to do the morning of your potential engagement. With that being saiddddddddd, it doesn't seem like much thought went into how you will be doing this.
Make it from the heart and about her. Make it special, it should be a day she will remember for the rest of her life....as one of the best days of her life.
Hopefully she says yes!
... and it will be NS wedding crashers this summer.