This web page by Giorgio Carboni explains how to easily and inexpensively project the view outside your window onto your wall by turning your room into a camera obscura.  All you need is some duct tape, posterboard, and a lens.

...it's best if the window faces a view with some activity, it can be entertaining to see cars moving and people jogging upside down across your wall.

Link

From the Upcoming ueue, submitted by Rob H..