I usually put together a page with photos of the ships we've previously cruised on and photos of "those we left behind" namely our dogs. You could post a theme based set of photos or pictures of your family, your home, kids hobbies, or anything else. It's really fun for the rest of us to look at the little "stories" on the doors. I bring scotch tape to put the collection on our door and have not had a problem with getting the tape etc off at the end of the cruise. We also like to post a map (showing the cruise route) on the wall inside our room.
I've also noticed that groups sometimes have identical flyers to post so they can find each other. This was all on Princess and I have never heard anyone speak badly about it. More than likely, others will say that they wish they put something on their door to find it easier.