Apart from having lots of fun you can also use LEGO as a medium for rapid protoytyping or holding various parts of a project together. One neat example of this has been demonstrated by Mathias Leroy whose Arduino-controlled demonstration of a solar tracker is integrated with the toy bricks.
The device has a light sensor on each corner of the square base - and the Arduino constantly measures the values from each sensor with which it can determine the direction of the strongest source of light - then rotates the "panel" using the two servos. This would be fun as a demonstration of a real solar tracker for a school science project or even adding to your own LEGO layout. Check out the following video for a quick demonstration:
If you're looking to learn about Arduino development platform and how it interacts with external devices such as servos and light sensors - you can't go past "Arduino Workshop - A Hands-On Introduction with 65 Projects” by John Boxall.
Arduino Workshop takes the reader from having zero knowledge about the Arduino platform, electronics and programming and leaves them with the know-how and instructions on everything from blinking an LED, to robotics, wireless data, cellular communications, motor control, sensors, Internet connected systems and more. For more information including a sample chapter and table of contents, visit the book page.