The easiest way to add a theme into a design is to add a plastic toy as a topper. In a princess-themed party tiny tiaras or small pictures of the loved princess can be added onto toothpicks. Even more entertaining and interactive for the children, plastic rings as toppers will give each child a ring as a party favor.
Besides the lesser cost compared to a birthday cake, cupcakes are a great alternative because children can help out in their construction from start to finish. It's less messy when little hands are involved. You also don't have to worry about cake plates, silverware and all the extra trash.