Message in a bottle guest book (people can write messages of their well wishes, or a piece of advice, or answer a question you ask them. For example, what is your favorite memory of the bride and groom or what should we name our first child, etc.
This is how I plan on asking the maid of honor & bridesmaids. Really cute idea. 5 Creative ways to ask: Will you be my bridesmaid? This photo is a DIY balloon in a box that you have to pop to find a secret message inside!