Let's Make Robots!

Simplest Robot project involving Arduino Uno

Hi Guys, 

Just got my arduino uno and i've completed the basis projects (manupulation with leds and piezo buzzers).

I really want to do something more serious..

so what is the simplest robot that i can start with?

something that doesn't take much time to build and doesn't cost much ... :)

I am sure this question was asked before, so excuse me for the repetition :)