Just to clarify what ElmhurstNick stated, dependent upon your status with National will determine the type of vehicle you can use the coupon for. If you are Executive Elite, you can use the coupon on any car class.

Regarding one-way rentals. I always need to remove my corporate code (can't speak for other codes) so I get unlimited mileage.

Try booking the reservations a couple different ways...no code, with code, etc.

As a couple others have mentioned, a free day isn't a free day at most locations. You will have to pay airport fees, recreation fees, local fees, etc. O'Hare is one of the worst, and surprisingly Pittsburgh was $20+ a day using a free day.

Good luck!