Nagai Hideyuki uses two sketchbooks to create 3D illusions. I was thinking this may be possible to do on the street/buildings with chalk, but in order to view it correctly everyone would have to be the same height, so that probably wouldn't work.
"I love drawing on graph paper. It's reassuring to have some gridlines to start from rather than a completely blank piece of paper."<--totally true! I'm really enjoying browsing Amanda Wright's sketchbook on Whit & Whistle.