A camera! Better yet, find a spot with a great background. Set up a sign like this and leave the camera (at your own risk of course) for loved ones to snap away. Leave a box of funny glasses, boa and more to see how silly they'll really get. DIY - #photobooth
Guest Book by lucyblueart, via Flickr would be super easy if you had a photobooth at the wedding (ie Chuckles) just add one of those cheap photo printers.