You got an A+, huh?
Nice one. If you give it more stages and pluses (some powers, for instance) and increasing difficulty you can do a real fun game. Say, have granny go faster but have her taking a friend to bingo in stage 5+ which is as slow as granny and follows her pace; granny being able to not just sort but also "destroying" them (have sausages for the dog, say...).
Just ideas if you want to keep it going...