This great sonar ranger from Devantech offers a wide range from 3cm to 3m. For further range see the SRF08. The SRF04 is interfaced using the one output line to ping the sensor and an input line to read the echo.
This is the SRF05 Ultrasonic ranger made by Devantech. The Ultrasonic detector can detect from ~4 inches to ~13 feet.
The MaxBotix low voltage MB1010 LV-MaxSonar-EZ1 offers very short to long-range detection and ranging, in an incredibly small package with ultra low power consumption. The MaxSonar-EZ1 detects objects from 0" to 254".
This lightweight but durable switch mounting bracket can be installed to keep your on/off switch firmly in place.
R19 is a low cost entry to bench top robotics, fast, accurate and reliable and easy to program. It is of the cylindrical format with four axes ideally suited to working in a plane around the robot.