Take a Kinect, capture topographic info and translate to a grid of fast-responding actuators to reproduce the 3d shape. Obviously limited to rolling landscape forms, but nice to see it in action. Basically, a robotic version of the classic pin-impression toy.
What if each column was made of individual cubes, and each cube could slide side-to-side in the XY plane? What if you made each column out of a flexible cable, able to be bent and manipulated by electric current?