Rice paper cake toppers
I saw no degradation in image quality (custom printed ones) in a few days. If the icing is hard then you moisten it just enough for the rice paper to stick before letting it dry again you should be fine as there's no source of moisture to make the ink suffer. But of course check the instructions - I think mine used alcohol-based inks. 

But why not ice the cake in advance then add the topper the day before serving?