Simple hands free inputs

I'm working on a simple light show sort of thing for a guitarist friend. Aside from preset knobs, I'm trying to think of simple ways for him to interact with the device to trigger modification. First thought was sound input, I just want a very vague sense of decibels, could this be done with just a piezo buzzer? Another idea was using an LDR for foot control but I don't know what the light conditions will be, is there some other way to get a virtual wah pedal type input without getting really complicated? Any and all other ideas on hands free input would be very appreciated.

Thanks

I think Hall effect sensors are used in some commercial pedals.

http://arduino.cc/playground/Code/HallEffect

I was thinking of that, but would it require him to stick a magnet to his shoe? Could I get some good resolution with a few inches of proximity, instead of a very tiny range of sensitivity?

Ah, you don't want to actually have a pedal. I didn't realise that.