Here's something neat I discovered while playing around with my new ball peen hammer. After hammering a coin and making it bend outwards, the indents from the ball peen make little plateaus on the other side. By sanding this side of the coin, you can take off the edges of these plateaus and show a branching pattern. Pennies look spectacular with this, the zinc pokes through out of the copper coating and shows a great contrast. the only trouble with pennies was that there wasn't much metal to work with. the end product turned out very thin, and kind of warped. I annealed a nickel and random arcade token i had before I hammered them, and the color from the flame stood out quite nicely against the brighter insides. I'm not sure how these would take polishing though.
The ideal procedure would be to first flatten the coin on an anvil or hard, smooth surface with the flat side of the hammer, then really go in and hit the back for a while, making it bow outwards. After this, I just sanded the outward face of the coin with some sandpaper. Unfortunately, I lost my really fine grit stuff, and had to settle with some pretty rough sandpaper until I get some more. However, if the time and tools are at your disposal, you can do a similar process to Mrballeng's Copper and zinc pendant (http://www.instructables.com/id/Dimpled-Pendant/ Major props to this guy, he really inspires me) and solder a nail to the back for circular sanding and smoothing on a drill. You would have to sand pretty lightly on the face of the coin, but this would be ideal for the edges, as the hammering process tends to turn the shape of the coin into something that only vaguely resembles a circle.
I've made a couple of these things now, and they're great if you are a beginner and are just getting in to metalworking. You could definitely drill a small hole in the top for a jump ring if you are making a pendant or charm, I just like them as something nice looking to fiddle with and carry around as a good luck charm. The design on these look reallly interesting, almost otherworldly, and they only cost you about 1-25 cents apiece. Anyway, if you do choose to try this out, have fun Hammering!