Experienced cruisers know the secrets of what to pack and what not to pack.  What are some of the "must have" things that sounded strange, or were though useless, but you're glad you bring them?

Sure, there's dozens, nay, hundreds, if not thousands of youtube videos and checklists of cruise essentials you can find all over the internet.  What are some of yours? 

One of the things I thought I'd never use or need is a shoe holder that hangs on a door. It's great for putting stuff like suntan oils, toiletries, wet flip flops etc.  A friend told me about it, and I though she was crazy but I indulged her and brought one.  I'm glad I did. it was one of the best ideas ever. Some cruise ships are not known for their spacious storage areas, so this is always a "must have" for me and has been for years. They fold up small, and are inexpensive and can be left behind if needed.