Not everyone, by a long shot, will participate in the fish extender exchange, and that is perfectly fine. For example, Andrew traditionally does not because of luggage issues coming from the UK. It's just one of many things people can do, and once we have official sign up, people can decide whether or not to do it. On the 5 night cruise following this one (which I am also on), there are over 130 families on the list, so it is simply not true that only smaller Disgroups do this.
People get very creative if they want to do this -- you can make small bookmarks or magnets featuring the logo from the trip, for example, with the only cost being the computer ink and paper and laminating paper or magnet paper. These are also easy to pack. Others will bring inexpensive stickers and buy bags of candy at one of the ports and distribute a sticker and a few candies.
The fun is in planning what you will give out, and in checking the fish extender when coming back to the room. If it seems more stressful than fun, don't sign up and don't worry about it!