Tell you have a companion ticket to Spain but she will have to travel a day later than you. Secretly travel the same day as her - get seats next to each other. Get the airline to sneek you onto the plane pre-boarding and be waiting at your seat with the ring and a couple of glasses of champagne (airline should surely help your plans and you might even wangle an upgrade).

Good luck and I wish you both many, many years of happiness ahead.

What airline are you going to be on. This thread might be about to generate a lot of interest as it has already been posted on Twitter by Flyertalk!