Take construction paper in red, white and pink (or one of the colors I mention) take scissors and cut them into hearts. Write little love notes on them. Like the kind on candy hearts or make up your own.
Then take them and put them all around the house. Like in the sock drawer, pocket of a shirt in the closet that you know they will wear, wallet, inside the fridge on an item they will pick up, coffee pot or mug, on the mirror, on the tv or computer. Where ever you think they will find them.
Make sure you make a lot of them. Have the kids go on a heart hunt. Give a small prize at the end of the hunt.