Assuming all efforts to return the money failed..and after a period of 2 years when statute of limitations ran out, I'd invest most of it and then take a well deserved long vacation in an area where the word 'snow' was foreign!
Side Note: A co-worker many years ago had $5,000 deposited into her checking account from her employer's bank. It wasn't owed her. She was on a expense account, so it was not that unusual for that to happen. What was unusual was the banks refusal to take the money back.She called her employer's legal dept. who advised her to deposit the 5grand into an CD, and turn over the 5grand whenever the bank finally got it's head out of it's ass. The interest it earned was hers, legally, as she had certified letters proving she tried to give the money back. It took 3 years for them to ask for it. Guess what had expired 1 year earlier?