Here's a cake a birthday kid will feel lucky to have. Festive enough for any spring or summer fete, it also lends itself to a science party. Just start with a frosted 9- by 13-inch cake, decorate with a handful of colorful candies, and get ready to sing "Happy Birthday."
Rainbow: Starting at the top, arrange double rows of red, orange, yellow, green, and blue jelly beans to form an arch across the cake. It looks best if you place the candies end to end rather than side by side.
Clouds: Mound a bunch of mini marshmallows atop each end of the rainbow.