We love this theme because it lends itself to all kinds of simple design ideas. Brightly colored balloons, circus music, popcorn, stuffed animals (or real ones), and bright primary colors are all you really need to make it work. Circus-themed birthday supplies are easy to find, and if you're hiring a clown or dressing up like one, so much the better. This is usually considered a theme that works best for younger children, but if there's a circus in town, your tween or younger teen will probably be onboard with it, too. If you want a colorful cake with a circus theme, try a Carousel Cake. Tip: Very young children may be afraid of clowns, so use some caution with the greasepaint and orange fright wig.