This wasn't free to build, but depending on what you find, you may be able to make it for close to free. I spent about $100, total.
This instructible is more of a "duh, why didn't I think of that?" kind of thing, rather than something really creative. But my kids were impressed, so I decided to share.
Step 1: Supply List
Small battery operated, motion activated light
Rubber door mat with "nubs"