This detector works by flashing an LED and comparing photocell readings with LED off and on to subtract ambient light and determine if there is an object in front of the robot which the LED light is reflecting from.

Since I already have photocells on my robot, all I have to do is add a high brightness LED. The one in the video cost about $1.25 at Radio Shack.

Anyhow, if you put your hand in front of the robot, it will now stop. Alternatively, guide posts could send it off in another direction. I can think of lots of ideas here, but what would be interactive and fun?