365 reasons why I love you jar. Both have their own jar and write once a day one thing they love about the other. On their anniversary/birthday/valentine's day/christmas/whatever the occasion, open the jar and read what the other person wrote
I love you clothes pins: Write a message with a FINE POINT sharpie so it doesn't smear. Write "I love you" on a paper. Cut it out, and hot glue it to the clothes pin end. Would be cute for a care package!