A heart cupcake.
All you need are some marbles, but a ball of aluminum foil would work just as well.
I made the hearts from blueberry muffins. Not very valentiney, but I was making some anyway and wanted to see how they would turn out. My only complaint is that the bottom is round and not very heart shaped, but decorating the top with a colored sugar heart (or frosting...but not on a blueberry muffin. Ewwww. OR maybe not eww?? Hmmmm...?) helped to make the heart shape stand out. Also don't fill the baking cups too high with batter because if it rises too high over the cups, they will fill in at the gap and look more like a blob than a heart.
Isn't that fun?! And easy!