Grab an old snapshot of you and your sweetie and put it on their pillow or on their steering wheel with a note attached.
On the note simply tell them how fun it was during that particular day or event. Let them know you remember it and you treasure that time spent together. The more details you can remember the better!
End with something like 'it's still as fun as when we first met' or 'just as in love with you as I was that night!'