I have used and re-used this craft program at my library. It is always a huge success, even with teen guys!
Bottle Caps rinsed with water and dried (ask your co-workers to help you collect some)
although having an assortment of different brands is cool, if it may be an issue at your library that some caps are from beer, you can lay them out and hit the backs with a quick coat of spraypaint.
Industrial strength glue
Collage Items (colored paper, magazines...)
Save some fun clip art or slogans on to your computer as jpeg files. Print them out using the thumbnail setting. This will make them small enough to fit into the bottle cap. They can also cut small pictures out of magazines.
Brush the Mod Podge on to the paper and place inside the bottle cap. You may also want to brush a layer of glue over the intire image for protection.
Using the industrial strength glue, glue a pin on the back.