One of my favorite craft projects that I have done with children is this melted crayon heart. Last year I was in charge of the craft for my son's second grade class Valentine's Day party. I searched for a fun craft that both the boys and girls would enjoy and when I came upon this one I just loved it.
Stand up card
There are two sources for this craft on the web - Disney's Family Fun and Martha Stewart. The Family Fun site gives you the directions to make the heart into a stand-up card, which is what we did. Martha's site makes them into pretty stained glass hearts to hang in a window. Which ever one you choose you will love the final result.