We recently learned about a practice of wedding guests signing stones instead of a guest book. The bridge and groom can keep the stones in a pretty bowl and treasure them forever.
You probably already knew about this. I'm a little out of the loop.
We have a beautiful alternative to offer: Signing Hearts. They provide a little more writing area than a stone would, and the marker will adhere well and easily to the frosted surface. They come in white, pink and lilac, in a set of twenty-four. See them here.