I am a big craft beer geek so I leave a bottlecap in a 2"x2" ziplock with a note with my handle and directing the user to my web page http://doppelbock.net/geocaching to find the picture of that cap and a link to the brewery's web site. I wrote up a quick article with images here: http://doppelbock.net/2013/12/05/geocaching-signature-item/