The other day I was googling Valentine's Day crafts for something to do that afternoon and I came across this one (I haven't found it again, but I think it was on a Martha Stewart site). It's really simple and only takes a few minutes. Basically all you do is iron crayon shavings (to melt them) between wax paper and then you can cut out cute hearts from the sheet. Then you can decorate your windows with them. I couldn't find any string, but it looks great if you hang them using string.