Paillette buttons

One-hole paillettes can be used as buttons. I've used mother-of-pearl ones here:

You might check bead stores for this sort of thing (although I think these came from Farmhouse Fabrics).

I sewed each on with a clear seed bead, just big enough to keep it from falling off.

Of course, you could use something intentionally contrasting or showy, too. I'd tend to avoid anything much bigger than necessary, though, because it might start to get wobbly or let the thread show.