Book the first night at this prepaid rate. Then book the next night on the lowest fully refundable rate for the same category room. Make a note in the comments box on the second booking of the confirmation number for your first night booking, and that you'll be using a 2nd Wkend Night certificate. Print out both confirmations and take them with you to the hotel. When you check in, inform the desk clerk of the reservation you have for the second night, and ask that it now be cancelled and your certificate used for another night in the room you've prepaid for. [Just make sure you can cancel the full rate room by 4pm of the day the booking is for. Or by that time on the day you check in. Some European properties have a 24-hour and even 48-hour cancellation window.]