Play with fire! Okay- this one needs some parent supervision. Light a birthday candle, and practice until the candle burns out.
Practice sprints*: Practice for 10 minutes,then put the instrument down and spend 10 minutes goofing off. Practice for 5 minutes, then spend 5 goofing off. Practice for 3 minutes, 3 goofing off, and 2 and 2. The times can be adjusted however necessary, keeping the spirit of the game. Another route is to practice for 15 minutes, spend 10 away, practice 10, spend 5 away, etc.