In addition to the difficulty of producing only very small motions, piezo devices - for useful mechanical output - also require rather high voltages. A tap on a small one, from a lightly spring-loaded "hammer" is sufficient to produce sufficient spark to light the burner on your camp stove, and to produce a motion of .008 inch per inch of crystal thickness (about as much as one can get without cracking the crystal?) may require on the order of 1500 V or more.

I can add a stock of dielectric insulating gloves (and maybe vests) to my store of asbestos ones - if order quantities are sufficient.

Alternatively, or perhaps additionally, small neon bulbs can be attached with the second lead simply hung out as an "antenna" so that the light flashes from the parasitic discharge when the device operates, much as some CB radio operators (a.k.a. "truckers") used to do so that their antennae would blink when they keyed to talk.

John