I wanted to hide and contain my cats' litterbox, help remove litter from their feet, and hide the kitty litter paraphernalia.
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
"Suncast 50 Gallon Deck Box with Seat" from Wal-Mart online
Small battery operated, motion activated light
Rubber door mat with "nubs"