Here's a simple, but fantastic looking idea using melted crayons and wax paper to make stained glass hearts that make a great sun catcher hanging in a window. However, you'll need an adults help with the ironing.
Beforehand, make lots of crayon shavings using crayons and a pencil sharpener.
Take a large sheet of wax paper and fold in half.
Open up and sprinkle an even layer of crayon shavings inside. Pastel pinks and reds generally look better for Valentine's, but don't be afraid to mix your colors.
Close the wax paper and fold a little down on the three other sides so the melted crayons don't squish out the side.
Lay down newspaper, the wax paper, and more newspaper on top. Iron with an electric iron on low to medium to melt the crayons. Don't make it too hot or they'll burn.
After melting the crayons, trace various sized hearts on the wax paper and cut out.
Tape a piece of string and hang up. Or, take many hearts and tie them to a coat hanger or paper towel roll to make a mobile.