On our last cruise, there were quite a few stateroom doors decorated for anniversaries and birthdays....the birthday doors had large birthday balloons (not inflated) adhered to the door with streamers, etc. I don't know if the cruiselines frown on this but lots of people do it. I think as long as it doesn't interfere with foot traffic in the corridor, deface the door in any way or hinder the room steward from their tasks, they don't interfere.