Other things get used too; buttons, random hammered pieces of sheet metal...but bottle caps are a favorite. Both of mine have them.
Backstory: to mostly Western ears, the buzzing can sound can be a difficult thing to adjust to; everyone wants to hear just the pure tones of the keys. But to the Shona, it is absolutely essential; necessary to help address the spirits of the ancestors.
The theory of Shona Mbira music is pretty fascinating. There are many, many set songs; none have a beginning, and none have an end; they are infinitely cyclical. Part of the theory includes the rule that the songs never actually end at all, and when one plays and sings a song, one is only joining a song that has never stopped. Improvisation is integral, but everyone knows exactly what's being played too; similar to a lot of Indian classical music in that regard.