We were initially going to make a photo book as our guestbook, but didn't have enough time to create one. So we went into an art store and came across a photography-themed journal filled with illustrations of cameras, photo-taking paraphernalia, and blank "frames." It gave us the idea to ask our guests to draw self-portraits. We allowed lots of space for our guests (many of them artists) to fill it in with their artwork and signatures and colored Sharpies added to the fun.
Plenty of guests did ignore the self-portrait request, but we were truly surprised, touched, and tickled by those who obliged. Even shaky stick figures and primitive smiley faces brought tears to our eyes as we read through the entries.
Obviously, any journal, scrapbook, or even book can be used, but you'll want thicker pages if you're using Sharpies to avoid bleed-through. Also, be sure to put up a sign asking guests to draw a self-portrait!