Something from the heart letting them know how special they are to you or how happy you are that they found each other. Wish them well, tell them congratulations, etc. It's kind of like signing a card lol.
You could be really different and sign it up the page vertically "some sign the front, some sign the back, let me be the first to sign your crack" haha who knows, this day and age with the time that most marriages last, it don't matter what you write because they will probably be divorced by the time they read it
something you mean. how happy you are for them how special they are to you. make it warm and heartfelt and then add something a little comical because then they'll remeber the nice things you said because you added a lil something extra
your name, and if u want to write anything else, maybe love ya or love you depending on how well u know them or just a congrats or something to that extent. i just normally sign my name.