Even after the city sold CEMUSA the right to build ad displays on every block of NYC, 99.9% of the city's surface area is going to waste, selling nothing. With ever more powerful portable video projectors coming on the market, the time is right to start your own pirate Ad Projection agency.
Get some clients and burn their ads onto a DVD. Rent a portable projector and mount it on the rear deck of a rental car. Park near large buildings and project ads onto them. Move every half hour to stay ahead of the police.
Bonus suggestion: sell ads to a name brand's competitors, then project them onto the brand's headquarters (below).
(Of course later I found that a few companies already do this, but not in a pirate way.)