Another Summer Activity for the Kids - Make Their Own Shower Curtain
If the kids have their own bathroom (or even if they don't), give them some fabric paints and permanent markers, and let them design and create their own shower curtain. I did this long ago with my boys, using a plain white flat sheet.
(The picture is a representation, and not the actual sheet they made. Theirs had mutant ninja turtles and such on it. :)