A "FISH EXTENDER" is a place where other Dis'er can leave treats...

We usually take a bag of candy, and my son and I will walk around and look for Fish extenders and leave candy in them. We received all kinds of stuff on the 11 day....candy, note cards, pirate patches, door magnets, beads etc...
We had over 120 fish extenders on the ship...
The one my wife made has four pockets, each pocket is about the size of a post card, and she decorated it with Disney pins and
DCL pins. Some people will put their name and kids name on it, it's and easy way for the kids to find their rooms....On the 11 day cruise, they even had a 'FISH EXTENDER' contest. A couple of the ladies, gave out award ribbons for the top 3 designs. DCL will also put your mail in it...plus it's a easy way to find your room. I will try and find picture of one and post it here.
Hope this helps....