Optical illusions
These are all very cool and almost immediately visible.
http://www.michaelbach.de/ot/index.html
May I suggest starting with these three? They're all great examples:
http://www.michaelbach.de/ot/mot_mib/index.html
http://www.michaelbach.de/ot/col_rapidAfterimage/index.html
http://www.michaelbach.de/ot/fcs_hollow-face/index.html