Adafruit has shared a bunch of cute DIY wearables you can make with Circuit Playground Express. Their latest project shows you how you can make servo actuated costume ears that perk up when there is a loud sound. They used Circuit Playground Express’ on-board microphone to detect the noise.
You can get the parts for such a project for about $50. You will need 2 servos and a LiPo battery in addition to a Circuit Playground Express.