We've mentioned it before, there are very good reasons to learn about computer vision, even if just as a programming exercise. Moreover, thanks to the wonderful Raspberry Pi single-board computer, it can be done for cheap!
As a fun way of interacting with the physical world, you can set out to perform face detection in real time.
Face detection is a computer technology being used in a variety of applications that identifies human faces in digital images.
Hackster user MJRoBot has published an excellent step-by-step guide on the site. The tutorial explains in great detail how to use a PiCam to recognize faces in real-time using the Python programming language.