I have a spectator take a quarter out of their pocket, memorize the date on the quarter and mark it in some way to make sure Im not switching it. I have them try to bend their quarter with their fingers. I then say "Hand it over .. I've been going to the gym every day to practice for this". Amazingly, they watch as you bed their quarter with your fingers right in front of their eyes (and yes .. you are actually bending it .. no switches). Absolutely astonishing effect.
Trick: Need a device called "superman" which can be found in most magic shops. It is two finger-looking metal objects with slits at the end (for the ends of quarters) that hide nicely in the hand while you're bending (nothing is visible at all). I usually throw the quarter on the ground after (gives me chance to ditch device) and have the spectator go look at the date and marking!