Things to do: Do your own ideas. You can animate the background and you can animate the fish. If your going to animate the fish you can turn the fish into a sprite instead of a backdrop. Things not to do: Don't be sad if you lost, because it's OK. ... Ideas: Spongebob ;) , "Cat/Dog" Fish, and Memes.
CREDIT: Scratch Programming, Music, FlockMod, and @blobbyfish123 for the script of the fish that follows you.