Conductive rubbers can be used for pressure, bend and stretch sensors. Because of their materiality they offer a nice haptic feedback, without the need for other squishy materials.
Images Stretch Sensor
Images, Scientific Instruments sell stretch sensors made from a resistive rubber that increases in resistance when stretched.
Zoflex offers two different kinds of resistive rubbers (ZL45.1 and ZL60.1) that work great as pressure sensors. One of them is more brittle and will rip if stretched, the other is more resilient.
>> http://www.irmicrolink.com/Pressure_activated_Conductive_rubber.html (USA)
Black Resitive Rubber Bands
also: ESD rubber bands
Good for use as pressure sensors, less reliable as stretch sensors.
>> http://www.all-spec.com/1/viewitem/1862C/ALLSPEC/prodinfo/i=rss (USA)
>> http://www.malaster.com/crb.htm (USA)
Flexible Stretch sensors
Resistive Finger Caps
also: Anti-static finger cap, ESD finger cot
I don’t know what these are originally intended for or who sells them, but they are made from a resistive rubber and possible to use them as pressure sensors.
Intended for use underneath sensitive electronic project assembly, these mats might have some interesting pressure sensitive properties.