(I've never actually done this, but I think it could work well.)
Need: -String -long poles/sticks -magnets -paper fish - make these on your own! -glue
Tie string to the end of a pole and glue a magnet to the end of your string. Next, glue more magnets to your paper fish (magnets as eyes?) and scatter them in a dry bath tub. Then grab a friend and go fishing!
PS. The set up is half the fun - so make some crazy looking fish :)