We are doing two! Mainly cause FH and I had different ideas we both loved and didn't want to budge. We are doing Jenga with a sign that says "Build Memories with us, sign a Jenga piece!" (or something like that). And polaroid pictures with a sign that says something like "We want to share these memories with you. Please leave a picture for us, and take the other with you one to remember tonight!"
We'll be doing a round one like this for signatures and then we'll do a separate thing for people to write their well wishes and advice. We'll have strips of paper with twine tied on the ends so they can roll the paper up and tie it. Then we'll have a glass jug or mason jar for them to put their notes in. Example below.