I Like Yellow - a Fish Extender is like a fabric pocket that you hang outside your door and do a gift exchange with other people on the ship (organized in advance, usually through the forums on disboards.com)
Anyway, we got a lot of magnets in our fish extender that could be used to decorate the door. Many were homemade, using clothespins and bottle caps. When I was looking for ideas, I went on Etsy and searched for Fish Extender. One that I saw and really liked was a Disney Cruise bingo game - it said either "Magic" or "Dream" across the top and and had 25 boxes with different things you could do on the cruise, both on board and in ports. It was designed to have one for each family member so they could compete to fill their game boards. This isn't the exact one I saw, but I like this one a lot, too: https://www.etsy.com/listing/183563606/instant-download-p...
Another great one, though certainly much more expensive - someone actually gave us an 8 x 10 picture frame mat and sharpies that we could turn in to guest services and get signed by the characters (in case you're not aware of that program, you can get up to two items signed per room and it doesn't have to be related to the fish extender at all). That was a pretty cool thing to get.
I just remembered a couple of other things we got and really liked - one was luggage tags that were brightly colored and stood out on our suitcases. The other one was fabric handle covers for our suitcases. We still use these all the time. They were very bright and made it both easy to identify our luggage and comfortable to hold the handles.
Some families gave one item total, while others gave one item for each member of the family. You have to remember to think about how much room it's going to take up in your suitcase, too.
Other people really like to give something that represents their hometown. If you have anything your area is known for, that can make a nice gift.
Someone else gave out mini, homemade scrapbooks with Disney-themed pages. They could also be used for getting character autographs on board.
Have fun!! We were supposed to go on another Disney cruise this summer and had to cancel. I'm so bummed!