Fun, unique sensors for musical toys?

Using Arduino Sensors
1

I'm putting together a series of about 10 Arduino-based synths, each using different sensors to make noise. I'm curious what other strange, cool sensors you all might be able to suggest for me. The sensors will need to be +5V powered and generate an analog signal between 0-5V (for use with the Analog In pins on the Arduino).

So far, I've come up with the following:
IR rangefinder (long distance) = general motion of visitors used for sound
Temperature sensor = thinking of casting these into small (<2" diamter) balls of ballistics gel, and the visitors have to pick them up and try to warm them to change the pitch
Alcohol gas sensor = your degree of drunk-ness turned into sound
Piezo element = after some AC->DC conversion, should be able turn force into sound (with lots of interesting echos and reverb from the fuzzy vibrational signature)
Flex sensor = will be slipping this into a medical tube, so that the amount of bend of the tube will determine pitch.

Can anyone suggest any other neat (and cheap) sensors?

2

Resistive force sensors:

Light intensity sensor also resistive:

3

Will definitely order some photocells. How can an FSR be used differently than a simple piezo element? Does it detect sustained pressure?

4

Force or pressure reduces the resistance of the sensor. You will need to make a series circuit of the sensor and a fixed resistor and sense the voltage in the middle. Google voltage divider.

5

Can anyone suggest any other neat (and cheap) sensors?

Every person is a resistor ... different parts have different resistance ...

Every person is also a capacitor ....

humidity sensor, radioactivity, hall sensor for magnetic fields, a rotary encoder (turning a wheel round fast) ,...
Have a look at the playground, a lot of sensors are mentioned there (including example code !)

my 2 cents,
Rob