For CSI-loving couples, consider creating a thumbprint collage guestbook.
Have ink blotting pads in different colours next to a framed image (and lots of wet wipes nearby!) and have guests create a fingerprint masterpiece! Ink pads are available from Peachwik via Etsy.com but you can easily create your own or check out Walmart or Staples.
If it is a large enough canvas, consider allowing them to sign their names overtop of their print (or initials) too!
Image: offbeatbride.com
Image: weddingswaterside.com
Image: weddingdaypin,com
You can find the basic prints (ie, bare trees awaiting the thumbprint leaves, etc.) from Peachwik.com