An extension activity would be a red fish, blue fish pairs match memory game. All you need is red and blue construction paper, markers and a little time! Cut out 10 fish in red and 10 in blue and write uppercase letters on one color, and lowecase of the same letters on the other. You can choose the letters that are most challenging for your children. Like any other matching game, flip 2 during your turn, if it is a match, it's your pair! 


Or, conversely, you could do the same idea with numbers instead, like in this example:


Or origami -

Here's the instructions for it all - https://artsycraftsymom.com/one-fish-two-fish-dr-seuss-craft-origami-fish/