This basic piezo sensor from Measurement Specialties is often used for flex, touch, vibration and shock measurements. A small AC and large voltage (up to +/-90V) is created when the film moves back and forth. A simple resistor should get the voltage down to ADC levels. Can also be used for impact sensing or a flexible switch.
Comes with solderable crimp pins and a mass attached to the tip. This mass increases the sensitivity to motion.