I don't know about in the cabin, but I know that it is forbidden to hang anything from the balcony. I would guess that also includes hanging anything on the exterior walls, but that is a guess.
You can put lights in your room, but they must be battery operated. You cannot put things on your balcony. I have used clings on the inside of the slider window.
People put lights on the hallway side of their stateroom doors all the time. I saw it on my Halloween on the High Seas cruise last month, and I've seen it on Merrytime cruises in the past.
Yes, they were up the whole cruise- that door was down the hall from my stateroom. And it wasn't the only door with lights on this cruise: see additional pics of different doors below- I walked by these every day. I've also noticed doors with lights on both of the Merrytime cruises I've sailed on.
Unless the rules have changed from Christmas 2019, we had battery operated fairy lights wrapped around pompom garland. We hung it up with magnetic hooks. I was planning on taking them for Christmas this year.
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