Is this a taxable windfall like a bonus from work? Or is it a non-taxable to me windfall like an inheritance? Because the answers will depend on that.
If it is taxable, I'd try to make some moves to minimize my tax burden - charity and what-not. You said all the retirement accounts were maxed out so there wouldn't be a lot of tax savings available there. But I might consider some back-door ROTH conversions or something that would be beneficial with a lump-sum (I have some existing IRAs that would be calculated into the conversion so I would end up with a tax hit and the windfall would be helpful in paying that tax hit).
After I took care of any tax consequences or tax beneficial moves, then let's see... I currently have a mortgage that I would probably put it towards. But you said the mortgage is already paid for. I would probably look into buying some rental property - commercial, multi-family or vacation probably. When I get windfalls, particularly bonuses at work, I like to be able to point to something and say "that" is a result of the bonus. Paid off a debt. Downpayment on the house. Bought a car. It makes it a little more tangible to me. Like I achieved something. So I would probably go along the lines of buying new property with a windfall.