Last year when I was working at my local arts centre I would sometimes work behind the bar for their evening events. I say 'bar', really it was three tables in front of two fridges but I still had to serve people drinks. The bottle caps would just get thrown in a box and then put in the rubbish so I often saved some of them, thinking I could make something with them one day. Well that day has come and I present to y0u the bottle cap barrette.
I started by drilling three holes around the side of of the bottle cap. You can put more for extra security when it comes to the sewing part but I'd say three is the minimum. (That's my Dad's model railway you can see there; I'm not a giant who enjoys doing her crafts on train tracks.)
I then made a yo-yo/rosette in a matching colour to the logo on the bottle cap. You can learn how to make them here. It's a German site but there are pictures.
Using a colour thread that matched the bottle cap, I sewed the cap to the rosette. It is a good idea to make sure the holes you drill are big enough to fit your needle through.