"What’s in it for the finder to be honest enough to hand in the item?" By asking this question, you deny yourself access to its answer.
In every situation, the most natural strategy is to maximize personal gains. That's how natural selection works. But when people don't act like that, it means there must be some opposing force, normally one that benefits society. There is a lot to learn here, in fields as diverse as biology, economics, computing, and of course, games.