Compass coins might be even cooler if they included an internal magnet, and could thus be used as an actual functioning compass. The existing spinner would work for this. After the top was spun, it would gradually come to rest pointing north (theoretically).
I don't know what effect the other metals would have on the magnet inside, but neither brass nor silver are magnetic themselves. A ceramic body could be pretty well guaranteed not to interact with an internal magnet.
Alternately, they could be made out of iron which is magnetised, and covered with an anti-rust coating.