I didn't return $45 to McDonald's after a mistake in tendering my money
Why didn't you return the $45?
I didn't actually need the money, so I just kept it.
I sometimes fantasize about getting free money too, so I definitely understand where you're coming from. But McDonald's is a business, and they would definitely notice such an amount missing from the register. Imagine if they made a mistake like this often, and nobody returned the excess change. How about you go ahead and go back to McDonald's, and return the money there was given to you by mistake.
Nah I think I'm gonna steal the money for myself. I can do a lot of things with it, so I'm not returning it.
I hope that you will reconsider. The managers of that restaurant are probably closely monitoring the register receipts, and an employee could get into trouble if they think that the cash was stolen. Since you don't really need the money, you can afford to be honest.