The stateroom doors are metal and guests are allowed to decorate using magnets. (No tape allowed.) We have decorated our stateroom door for each of our three cruises. This has helped us locate our stateroom and show off if we are celebrating anything special.
In the photo above, we found balloon cutouts at our local dollar store and decorated them with stickers. One balloon announced who we were, while the other told that we were celebrating a very special birthday. We used magnetic clips, also from the dollar store, to secure them on our door. The magnetic clips can also be used on the other side of the door to hold meal tickets, Personal Navigators, or any other paperwork you want to keep handy. (Horizontal space can be a premium in staterooms, so utilizing vertical storage means more room for everything!)
For this cruise, we used printed photos of us at Disney as part of one display and the other was a map of our itinerary along with dates we were in each port. We used magnetic tape to stick to the back of the photos and then posted them on our door. These were all pre-made prior to our trip, so all we had to do was display them. Voila! We would often discover other guests browsing at our stateroom door. And, we would often take various routes back to our stateroom just to view how other guests had decorated their doors.
So now it is just you, your imagination, and a bit of creativity able to design your own decorations for your stateroom door. I'm already planning for the decorations for our next cruise--using scanned images of things we have received in the mail. We have extra reason to celebrate, as it will be celebrating our anniversary on our next Disney Cruise.