National has Quick Rent where you book your car and then when you get there just head to the aisle of the car you booked and pick which one you want. Only down side, you cannot use a coupon.
But if you join their Emerald Club (free through a link at Mousesavers.com) then you can also bypass the counter and pick from the aisle you booked but you can also use a coupon. If you try and use an upgrade coupon or rent a carseat then you cannot bypass the counter.
I LOVE National!