We did three activities to go along with this book (hope you've got yourself some tin foil!)
I drew a simple fish shape and the kids used tin foil scales, glitter glue, googly eyes, and markers to decorate them. They also used the bingo markers to make "bubbles".
I attached some colored and tinfoil scales to clothespins and clipped them to my two little rainbow fish. We read the book and then I encouraged them to trade scales. (We have a way to go in the sharing department around here.) If you were doing this with a larger group of children, you could designate one child as the rainbow fish and only give the shiny scales to that child. As you read, the rainbow fish could give his or her shiny scales to the other fish.
I cut four different colored fishes from construction paper and made one the rainbow fish by adding different colored scales. Each got six matching scales except the rainbow fish, which got tinfoil scales. (I laminated everything but you wouldn't have to.)