Mathematics - Optical Illusions
As a former math teacher I liked using optical illusions. Students found them fascinating and I was able to effectively use them to teach several principles.
The site is called 72 Optical Illusions and Visual Phenomena
One of my favorites was the missing corner cube. You have to change your focal point to see the missing corner cube as an EXTERNAL object. Some students will get it right away, some will take a while, some will struggle. It makes for nice group work as students who see it try to coach and verbalize what one must do to see the alternative view.
I know many of our CFF coaches look for good math activities. Many of the illusions are interactive and quite engaging. I would definitely recommend this site. Do have java enabled in your browser.