Meet the ZENrobot: a robot capable of drawing circles, spirals, and other patterns in the sand. It can take up to 45 minutes to complete the pattern depending on its complexity. The robot starts clearing the surface as soon as it is done to try a new pattern. It’s not the fastest robot but watching it can be quite meditative.
If you know your own Arduino and don’t mind to code a little bit, you could always build your own sophisticated robots to draw on various surfaces. For this project, Processing is used on a laptop to generate patterns. More info is available here.