There are already plenty of fun iPhone apps that let you make music anywhere. AirSynth for iOS is pretty cool though. It uses your phone’s depth sensing camera to detect your hand movements in the air and turn them into musical sounds. You can use this app to simulate a theremin.
AirSynth provides you with a real-time visualization of the depth data the camera sees. Its depth control works in the dark too. Just bring a Face ID device, and you are ready to go.