You will see all sorts of decorations on stateroom doors, from nothing at all to so many that you can hardly see the metal anymore!
Some of the most popular ones are a pair of Mickey (or other character) ears (and sometime other accessories too) that go around the circular stateroom number near the top of the door. I think these are really cute, and you can even buy sets of these on the ship now. Some people also make personalized magnets, often including family names, their itinerary, the ship name/logo, etc. The sky is the limit!
My upcoming Alaska cruise is our 4th cruise with
DCL, but this will be the first time I will decorate my door. My parents are coming with this time, and I want it to be extra special. So far I've designed a few things. One is a rectangular cruise magnet, a bit smaller than an 8x11" piece of paper, that has our family names, the year, and the ports of call on a background photo of the ship in Alaska. The others are a Mickey Head for each person with the DCL logo, their name, and their favorite character on it. I also plan on deciding on some Mickey ears and or other character magnets just for fun.
I'm having a lot of fun doing the designs myself, but there are multiple people on this board that will personalize items for you as well.
Hope this helps!
Aby