This page shows a few ways to sense touch or force and convert it to an analog voltages.
1V/Oct analog keyboard interface
This is a vintage style analog keyboard interface that is used e.g. on the stylophone to convert touches to pitches in 1V/Oct. More sophisticated version with note priority and touch/release detection can be found on music from outer space.
Velostat Pressure Sensor / Moisture-to-Voltage converter
This circuit can be used to detect finger pressure or any force on a surface with the help of a pressure sensitive sheet material called 'velotstat'. You can find a tutorial how to build the physical sensor here.