This is a simple but capable distance sensor - able to sense a 30 degree range, using 2 IO pins on your microcontroller.
Like all the products on the Orionrobots shop - this requires no soldering to get it working, and it comes with 4 simple hookup cables to connect it directly to the Arduino so you can get trying distance measurement really quickly. It uses 5v logic.
This is what can be done with 2 of these, a few spare bits and an Explorer 1 kit:
Read this to work with it and an Arduino: Arduino Basics: HC-SR04 Ultrasonic Sensor
Time to build your own robot! Using a Raspberry Pi with parts and some time, you can use my book to learn how to make and program a robot with automatic behaviours.