At a wedding that I did the photography for the bride and groom had a quilt that had white strips within the pattern. They then had the guests sign on those strips. It was very cute and when the wedding was over the quilt was already done and ready for use/display.
Another idea, my husband and I used our engagement picture and had a large mat for folks to sign. I have it on our wall and it is still fun to see. Espically where my nephew signed, Mr and Mrs "Jones" and there were lots of relatives with that name! We smile every time we notice it. Oh, and yes, we did figure out which one signed it that way!