Wood floor inlaid with pebbles. This could also solve the problem of shrinking wood planks or uneven floors that eventually leave gaps in-between. Fill in the gaps with pebbles & it'll look like it was designed to be that way.
One of the few modern bathrooms I found with a penny floor. Since I work with more modern bathrooms anyway, this seems more realistic to complete since it's the smallest room. So that means the least pennies:) one day I'll get around to it!