You can do a lot of interactive project, the most commonly used is for sound and light shows.
Try to the experiment with digital sound.
Buzzer module make the simplest sound.
Just change the frequency, you can hear different sound (like alarm).
It is used on your daily appliance, like PC, phones and so on.
Power voltage 3.3V~5V
Signal input: digital (PWM)
10 x Buzzer Module
10pcs RobotDyn Buzzer Module 3.3V~5V PWM Digital Input Board For Arduino DIY Project