As for fish extenders, next to your cabin door, there is a fish that is sometimes used by the crew to place envelopes/messages:
Some people "extend" their fish with a set of hanging pouches for the purpose of a gift exchange with other families on the ship:
If you check out the main DCL board here, you will see lots of threads related to Fish Extender groups (a.k.a. 'FE'). Some people find it to be a fun way to prepare for their cruise, others don't really care for it (for various reasons).
There might be a group set up on that other social media site - I don't follow what happens over there so I'm not sure. It's getting late to get into an FE group, but sometimes organizers will put together a latecomers group.
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