If you plan to measure the distance you can always use a cheap infrared distance sensor like Sharp GP2Y0A02. It is really easy to make an interface to it using Arduino. Link to the project : click here
Description: This Distance Measuring project interfaces an application module with AVR microcontroller and two SFR04 sonic modules. Measuring principle is very simple. Measure time between sonic transmission and echo reception and then divide it by sound speed in this environment. The distance measured is displayed on LCD screen. This distance meter can measure distances from [...]