If you want something personal and exciting, that will allow you to reminisce on your memories together, this is a great idea. It will rekindle your love by allowing your significant other to "hunt" for your old memories. This idea is very personal and can take some time to prepare making it even more special!
You can add notes that give clues and hints, leading to the final destination and purpose: The proposal! You can put the notes in her purse, on the bed, or even send it with flowers.
You can have your first note, leading her to your first date spot, or where you met. In the note, ask her to wear what she wore on that date, (if she still has it or remembers), this can even be the hat, shoes, or something else that will remind her of that first date or first time you met. You can actually prolong the fun by doing this every day for a week to build up the suspense and excitement. Each day, you can leave another note, giving her more hints and clues to find old memories. You can have her search for different things you kept throughout your dating period that bring up great memories. For example, the ticket stub to the first movie you went together, the receipt from your first dinner, pictures you took together from your first vacation. It can even be a "hunt" of a memory you have with her.
You can give her hints and clues to remember certain things you shared as a couple. Like the first time you went to her parents house and ended up getting lost and showed up 1 hour late to dinner! During this time of searching and hunting, she will be very curious to know what you're up to and what this all will lead to. You can either tell her to be patient and wait and not reveal anything more than that, or you can tell her "this is the pre-party for the major event." Or "this is all going to lead to the major hunt." Or something like that. Your choice.