Lost keys?

Go to home depot or any place that still cuts keys for you, preferably not a place with one of those automated machines and ask the employee if you can have the bin with all the miss-cut or trash keys. Most times you can get them to give them up, even if you have to explain that it is for a prank on a friend.

Then buy a package of those cheap paper tags that can be put on keys, fill out all the tags with something along the lines of "Reward if found, please return to..." and put the victims name and address or even phone number.

For the next few weeks, every time you go anywhere take a pocket full of keys and drop them in random places.

Enjoy as the victim gets repeated calls or even envelopes of keys mailed to them.