I did some projects with my Arduino. First I started with just testing a diod and a switch just to get a hand of it, because it has been a while.
On the image displaying my Arduino I did the project Light Theremin from the book Arduino Projects Book. The black round object on the breadboard is a piezo which can make noises (quite annoying really). It is connected to a photoresistor, which senses light.
So what this project does, is that the piezo will make sounds depending on how light the photoresistor takes in.