It's common knowledge to most parents that, given a number of options, their child will, more often than not, choose to play with a cardboard box over an expensive toy. So, taking a cue from my two-year-old's favorite temporary exhibit at the local children's museum, we've been keeping all our bottle caps and covers in collections that make for excellent “new” toys on rainy days.
The covers are perfect for stacking and sorting, especially if you can round up a variety of shapes, sizes and colors. Or if you are feeling creative, you could paint them different colors (and get your child to help) or glue pictures to them, but even the plain old covers have inspired plenty of imaginative play in our house. We keep a small bin by the sink and toss all of our covers in after we've cleaned them and recycled the bottles, cans and jars that they came from. We also try to find interesting boxes and containers for the caps — which always get put away at the end of playtime — so that they don't start to spread around the house and become garbage again.
MORE BOTTLE CAPS ON OHDEEDOH:
Green Idea: Recycled Bottle Caps
More on "Un-Toys"
Images: Sarah Rainwater
Must-See Small Cool Homes: Week Two
Beth's Romantic & Retro Residence
Good Food to Share 
The Big Book of Small, Cool Spaces 
Apartment Therapy Presents 
The Eight-Step Home Cure 
Shaw's Original Fir...
How to Make Easy 3-Ingredient Energy Bars at Home
How To Cook Tender, Chewy Wheat Berries on the Stovetop
So funny to see this. My parents, wise grandparents that they are, save the lids to laundry detergent and fabric softener. My three-year-old daughter plays with them non-stop while she’s there, endlessly stacking, color combining, and whatever else she thinks of.
We started keeping caps last year, its amazing how many caps you go through in a year. The kids and their friends all love to play with them.
Please make sure they aren't small enough to be choking hazards!
What a great idea! As a new parent with a 5 month old, I plan on going home tonight and sorting through my recycle to start this collection.
I cut round photos of family and slip them in the underside of milk caps.... in pairs... and we play memory with them. So fun for the littles to find a match to a picture of themself, the dog or a loved one.
Also... the red ones... taped to the inside of a cardboard box... become the blast off button.
I also keep my bottle caps for 2 years now and yesterday my baby put one in her mouth and almost choked, be careful with little ones!
I used milk jug lids to make a memory game! http://sixgoldencoins.blogspot.com/2009/09/crafts-for-christmas.html