How many exactly do you have? Depending on the number, you could:

Drill small holes in them and stitch them to a shirt to make a scalemail shirt for the next Creative Anachronism faire.

They're fairly soft, so file off the back sides of two of the one-Yen coins, solder together what's left to make a two-headed coin. Great for winning bets.

Paste them on the outside of a small wooden box to make a cheap omiyage (souvenir) for yourself or one of your loved ones back home.

Pretend they are pirate treasure and bury them in the backyard. For more fun, make a treasure map and leave it lying around for some neighborhood kids to find. (But if you do this, make sure you bury the "treasure" in someone else's yard.)

Give them to an acquaintance who is on their own way to Japan, and see how many they come home with.

All it takes is a little imagination.