If you liked yesterday's tip about using puzzles to engage your learners, you'll enjoy using optical illusions. They're particularly handy for lessons about perspective, world view, convictions, bias, subjectivity, seeing things that aren't there, missing things that are--you get the idea.
To find free downloadable (or copy-paste-able) optical illusions you can try Cool Optical Illusions (when you get there, just click on any illusion listed in the sidebar on the left), The Optical Illusion Archive, Optillusion, or Eye Tricks.
I find the "Cool Optical Illusions" site easiest to navigate, but I've used illusions from the other sites, too.
Fair warning: in addition to illustions, these sites contain crass humor links, magic links, and tons of advertising, most of which I would not endorse. If you stick to the illusions, you'll be fine.