I used to work with a guy who cleaned a lot of metal car parts by letting them sit in a container of Coke. He said that you have to rinse them off really well afterward, otherwise the Coke will keep eating away at them.
Gardengnome, I don't know if Coke would make copper coins shiny again, but somebody told me that baking shiny new coins in the oven will cause them to turn interesting colors. I tried it and it works! It gives them a sort of rainbow effect.
Lexie, do you think it could eat the paint off of those plastic bumpers? It might be worth a try if you plan to repaint one anyway, unless it eats through plastic.