The GP2Y0A21 Sharp distance sensor is a great way to add obstacle avoidance or motion sensing to your robot or any other project. With a detection range of 4" to 32" and an analog voltage indicating the distance, this sensor is very easy to use.
The URG-04LX-UG01 is a laser range-finder (LiDAR) that's inexpensive, accurate and easy to use. This laser utilizes a USB 2.0 interface. The field-of-view for this detector is 240 degrees. Distances are reported from 20mm to 5.6 meters.