I'd find it hard to explain a sudden influx of money in my life :o)
Seriously, though, if there were some identifying information, I'd return it. Whoever lost it would probably be in some serious anxiety. Without identifying makrs, I suppose I'd hear for the local police department, have them inventory it in my presence, sign a receipt for it, and hope they find the owner. At least then you have some claim to it if it isn't claimed. A more paranoid person would tell the police they'd found a large sum of money and provide a point of contact so that the owner could claim it by some suitable method of proof.