Jetpack Joyride is one of those addictive endless games. You go as far as you can in the stolen jetpack, dodging obstacles, collecting coins, and completing missions. When you complete missions, you get stars; earn enough stars and you level up, from Beginner, through Hotshot and Ninja, to Barry (the name of the character).
So in my class, each assignment would have a star value. When students complete the right assignments, they can earn a new overall rank, e.g. Multimedia Ninja or Graphic Design Apprentice. There are also two levels within each unit for two levels of mastery, e.g. Architecture Pro or 3D Jedi. It would work even better to have a physical representation of these, like a sticker to put on their binder or folder, combined with a candy bar or a head of lettuce. (I can't come up with anything healthy that would actually be a prize.)