Challenge them outside school if you are not planning to change the status quo. You can do it with harder academics, learning a musical instrument to an advanced level or sports (martial arts is one such sport that teaches practicing a skill to perfection). Even better, you can do all of it.
The bigger problem with coasting, in my opinion, is that the child goes through schooling without knowing what it is to work hard for a goal and also never learns good work ethic.