Turn an old Geiger counter or CD meter into a prop, by adding an arduino and speaker to simulate clicks. I use a tilt sensor to activate that makes it hard for people to figure out what is happening.
In my projects I use a simple Arduino board, such as an Adafruit Itsy Bitsy, along with a small amplifier, and a small piece of protoboard to connect everything - Powered by 3 x AAA.
Typically I wire the original power switch as power on/off, and change the light bulb to an LED. The original meter is attached to an analog pin via a current limiting resister.
To activate I use a small tilt sensor wrapper in Velcro, and position it on the size so I can move it around to get the right spot.
When using the counter, I gently activate to get the "ticking", with the light and meter displaying. Gets a great response and is very simple.