Print thousands of unique puzzles (math, image, text puzzles) of varying difficulty on the windows of buses, trains, metros etc. with subtle, thin white strokes of paint like this. Each puzzle would have a code that would point to a website on which people could post their solutions. Everybody could contribute new puzzles that would be printed somewhere in the public after being reviewed and designed in a consistent way by an editorial team.
Print thousands of unique puzzles (math, image, text puzzles) of varying difficulty on the windows of buses, trains, metros etc. with subtle, thin white strokes of paint like this. Each puzzle would have a code that would point to a website on which people could post their solutions. Everybody could contribute new puzzles that would be printed somewhere in the public after being reviewed and designed in a consistent way by an editorial team.