score:5
Buy two full-fare tickets. They are typically fully refundable. Obviously, check before you purchase.
You typically buy directly from the airline.
Then, whether she says yes or no, get them refunded and buy something sane. You'll have to pay a small charge, but nothing compared to the fare difference.
Or -- and this is cheaper if you are going to just print the ticket yourself -- forge the tickets completely. Just take an old ticket and replace the dates and destinations.