As suggested in the archive circulated by Jim, I always come prepared with extra app questions (sponge activities) for groups who complete the app early. I have experimented with assigning bonus marks to them but am undecided whether or not that is a good idea. On the one hand, without the incentive of bonus marks, early groups don't necessarily apply themselves to the additional assignment (those intrinsically interested always run with it). On the other hand, it can make slower groups rush unnecessarily through the assigned app in order to have a chance at the bonus marks.