Colored papers, wallpaper samples, magazine pictures, wrapping paper, etc. Cut front panel of cereal box apart from rest of box.
Cut heart shapes out of cereal box board to make stencils for cutting lots more hearts.
Use your new cardboard stencil hearts to trace as many hearts as desired on the back side of selected wallpaper samples, colored paper, or whatever and cut out. Glue hearts on card to form desired patterns and let dry. You can also continue decorating on the inside of the card if you like. Don't forget to add your Valentine greeting and message to your special card.