This basic piezo sensor from Measurement Specialties is often used for flex, touch, vibration and shock measurements.
A simple resistor should get the voltage down to ADC levels. Can also be used for impact sensing or a flexible switch.
Operating Temperature: +32 to +158 °F (0 to +70 °C)
Power Requirements: N/A
Communication: Analog (Up to ~70 VDC; Sensitivity 50 mV/g)
Dimensions: .98 x .52 in (25 x 13 mm)
Wide range of applications
Thin polymer film laminate with 2 crimped contacts