Well, I would attempt to return the wallet. Assuming I wouldn't have to drive more than an hour or two in one direction, I would first go to the person's house to return it. Any gasoline I need for my vehicle would come out of the cash in the wallet, and I would keep receipts (duplicate copies, of course) to prove why I spent some of their money. I mean, I'm using my vehicle and my time to return their stuff, so at the very least they can pay for gas.
If I couldn't reasonably go to that person's house, I would make every effort to contact them and have them come meet me somewhere in town so I can return the wallet to them. Call me paranoid, but I won't give them my address to come and collect it from me. I live in a major city with a ton of crime, and the last thing I want to do is be the good samaritan that gets shafted trying to return someone's wallet.
If I cannot reasonably find the person on my own, the wallet, including all cash, goes to the police station.