The ThinkerShield is a simple electronic board that plugs into the Arduino. It provides six LEDs, a potentiometer, a light dependent resistor, a buzzer and a button. It also has 12 connecting points for alligator clips.
The board was developed for the NSW Museum of Applied Arts and Sciences and is used by many NSW schools.
You can order it from MAAS or another retail provider such as Freetronics. Google search
The board design is open source. You can use the ThinkerShield schematic to simulate it on a breadboard or, if you're electronically minded, have a board manufacturer create boards for you.