Good luck with your project! There are a number of sensors that you can use to measure distance. The Parallax PING uses ultrasound and is fine for detecting obstacles up to ~10 feet away. For longer range you could try interfacing a laser distance measuring tool to a Prop. Something like this:
http://www.homedepot.com/webapp/wcs/sto … 671265-_-x
Make sure you get a true "Laser" measure, and not a sonic one that just uses a laser to draw a dot.