Alternatives to Guest Books. What is everyone doing?

I was talking to a wedding coordinator this past week. She is a staff member at my daughter's day program who just got into the business and we were talking "wedding shop-talk" ... she asked me how the planning was going. I'm down to the small details now with about a month to go. She was saying that hardly anyone does guest books anymore. She threw out 2 options, something about a tree and sharpie markers ... and another idea that I forgot about (obvs I guess I didn't care about it to forget about it?) What is everyone doing as alternatives to guest books. I'm not having any luck searching this on my own.