Everyone has given great ideas!! I brought clothes pins and couldn't remember why, now I know, but I did use them to bind stuff together, but dummy me didn't think to use them on the curtains, and we really could have used them.
The other thing that came in really handy and was a lifesaver, especially if you like an organized cabin, is an over-the-door shoe holder. I use it to put all those little things that you keep on the desk area, or on the sink. I'm considering getting another and actually using it for shoes. Plus then I'd have more pockets. I'm just a neat-freak when I travel, I hate spending my time looking for lost items.
I also brought post-it notes and a pen, used it a lot. We have kids, so we brought walkie-talkies as well. The closet have a few hangers in them, so when I packed, I packed the hangers as well, I was very glad to have my own. I've read where people bought the inexpensive hangers and just left them on the ship, so they only had to pack them one way. We brought our own clock, I have this thing about knowing the time and not relying on my watch. Getting ready for dinner, or leaving on time for excursions.
I have used the laundry facilites due to an accident with clothing, and I was impressed. They were clean and plentiful. Not sure if I just hit it at the right time or what. It was in the evening when we were sailing. I did bring my own quarters, because we stayed in San Diego for 3 days prior and I did laundry before getting on the ship, but I see now that doing it on the ship if I had to would work also. So I'll be bringing quarters with me in the future. My goal next time is not to pack so much!!
Have a great time cruise!! They're addicting so watch out! 
Carnival Elation July 2008
Carnival Paradise July 2007
Carnival Holiday August 1993
Carnival Mardi Gras 1992
Carnival Festivale 1991
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