Well technically the light follower("photovore") is an autonomous robot. But If you want, you can upgrade to the Sharp IR rangefinder which will actually detect obstacles and avoid them(Or run full speed into them if you are making a sumo bot
If you dont have any sensors on your bot, he/she wont be able to know anything
about the surroundings and thus cant do anything but go in a straight line unless you specifically program it to go a certain route (ex. Go straight for 10seconds, Turn right, Go straight for 15 seconds, Turn left, Fire Death Ray for 5 seconds) that sort of thing.
You can put a bump sensor if you want. That way when it runs into something, it backs up for a second, turns, and goes somewhere else.