I was diverted today for some time by http://www.michaelbach.de/ot/index.html which is a trove of “visual illusions” and explanations for many of them. A lot require Flash, which I stubbornly refuse to install, but enough are presented as images alone that I was able to experience a lot of surprises.
You've seen the moving-without-moving blue and yellow wheel already perhaps, or the grid of white lines on a black background that seems to have black dots at the intersections, but only the intersections you aren't actually looking at. But have you seen the non animated image that starts out blurry and goes sharp, just because of what you were looking at before it? Or the instructions on how to draw impossible objects?
Look at this tiny animated gif from the main page. Think of it as flat. Now think of it as something that goes down into the page, like a crater or a bucket. Did you get a surprise?
I really must go do some work now.