A.L.
You could throw them a pretend wedding with a pretend minister (like a child in priest outfit -it's holloween I'm sure you could find something) to perform the ceremony - but still make it cute with a simple arch and some wedding bells, ballons to fill the room and a cute poem to welcome her new husband and celebrate their union.
Good luck..
PS...I like the idea of a box with one thing on it but has a nice gift inside....maybe it could be a fredricks of hollywood or victoria secret box - the may get red in the face for a few seconds before opening the box.