This large 30mm diameter piezo element is nicely enclosed with mounting holes so you can mount it easily on something.
Piezo elements convert vibration to voltage or voltage to vibration. That means you can use this as a buzzer for making beeps, tones and alerts AND you can use it as a sensor, to detect fast movements like knocks.
Grove - Piezo Vibration Sensor is suitable for measurements of flexibility, vibration, impact and touch. The module is based on PZT film sensor LDT0-028.