Back in the 1990’s there was this fad of stereographic posters, and you could see people standing in gift-shops, staring at completely abstract patterns and occasionally here someone crying “I CAN SEE!”
If you got the hang of it, you can move to the next level of stereographic videos: Just go to full screen view, cross your eyes and step into the third dimension.
This one requires more intense eye-crossing, but is well worth it:
The Daily Psychedelic Video 