One of my favorite ideas for the wedding was our guest book.  Typically people leave out a book for guests to write a few words and sign their name.  I feel like that would be something I'd look at once and store away.  I saw this idea using rocks for guests to write a message on and putting it in a vase.  I thought it was a really cute idea and would look great as decoration in our house.

I ordered 75 - 2"-2 1/2" river rocks from Etsy seller PNWCoastalCreations for $36.30 with shipping.
Me and my husband signed the first rock to get it started :)

It took me a while to get on this, but you should spray the rocks with a clear gloss after the wedding.  This will help seal them from fading over time and any dust or dirt.  But it did darken my rocks a little bit so I would suggest spraying as far as you can.