Everybody carries a set of keys with them. Sometimes the key ring will have a unique fob, but in general, they are very much the same as anyone else's keys will look. So it's not hard to come up with an identical set of keys.
Look at your victims key ring and make a note of the style of keys. Then make up an identical set of keys as their own. You can either swipe their keys and replace them with the bogus keys, or just leave them out some place where they will see the keys, assume they left them out and take the keys.
Now your mark will have two identical sets of keys. One which fits all of their locks and one which doesn't fit a single lock that they own. The pleasure here is to watch them go crazy trying to open their car with the wrong keys or lock the front door. All you have to do is sit back and enjoy the fun.
Or, another angle is to replace at random keys on their key chain with the wrong key.