Leave the key card on the desk. If you're concerned, call the hotel before checkout time to inform the front desk that you already left earlier in the morning. I wouldn't do anything to call attention to the fact that you haven't spent the night in the hotel or don't plan to do so (such as refusing the key card or immediately dropping it in the box). You might want to also mess up the bed and use a towel in the bathroom so that it's not real obvious that the room wasn't occupied.

Not all chains count day rates as stays or give night credit for such day use, so check the Hilton T&Cs carefully.