Can you find a small silver platter with maybe a domed lid?? Design a beautful gift certificate and put it inside the tray. Wrap it in clear cellophane with lots of beautiful shiny Christmas curling ribbon!~ That's be really cute!
Or maybe you could find a remote segway (like a remote car) and put the certificate on that and drive it to them Christmas morning.
If they're Disney fans (sounds like they might be) See if you can find a plush Mickey dressed in formal wear (Groom Mickey???) nad have him hold the certificate!