Although they don't employ an Arduino, "Herbie" style photovore robots are fun.

How to turn an old computer mouse and some electronics junk into a light seeking bot:
http://www.instructables.com/ex/i/941512DCE05410289F96001143E7E506/

Here is a slow one:

They can be made much faster, and they are lots of fun especially when you have an easily scared cat.