One of the projects I intend to make some day is a metal detector with 2 pickup coils(*), one in or around the sole of each shoe. Instead of an auditory signal, a vibrating element strapped to my leg or ankle. That way you can metaldetect difficult terrain/non-stop/inconspicuously.

(*) they each work independently, like two metaldetectors. Not the detector type with an RX and TX coil.