An adjustable potentiometer can open up many interesting user interfaces. With this rotary potentiometer just turn the dial and the resistance changes. This potentiometer from Bourns has a ¼" metal knurled shaft diameter, PC pin terminals, no detents, and has a 100K logarithmic taper.
Connect VCC to an outer pin, GND to the other, and the center pin will have a voltage that varies from 0 to VCC depending on the rotation of the pot. Hook the center pin to an ADC on a microcontroller and get a variable input from the user!