Tessellations

In this app, you choose from a variety of tessellation shapes such as triangle and square. You begin with that basic shape. You then get to change the shape by "drawing" with your finger. It's less like painting and more like drawing with a ruler. (Or, if you like, it's less like pixel drawing and more like vector drawing.) Of course, you do not have complete freedom to draw whatever shape you want because the shape must tessellate. The app does an excellent job of maintaining the tessellation as you draw. You also have control over the colors. It's really quite entertaining.