A photo interrupter sensor is composed of an infrared emitter on one upright and a shielded infrared detector on the other. By emitting a beam of infrared light from one upright to the other, the sensor can detect when an object passes between the uprights, breaking the beam. Used for many applications including optical limit switches, pellet dispensing, general object detection, etc.
Infrared proximity sensors work by sending out a beam of IR light, and then computing the distance to any nearby objects from characteristics of the returned (reflected) signal.
Piezo vibration sensor
The Minisense 100 is a low-cost cantilever-type vibration sensor loaded by a mass to offer high sensitivity at low frequencies. Useful for detecting vibration and 'tap' inputs from a user. A small AC and large voltage (up to +/-90V) is created when the film moves back an forth. A simple resistor should get the voltage down to ADC levels. Can also be used for impact sensing or a flexible switch.
A pressure sensor may be a trigger activated by weight.