Learn To Use Your "Other" Hand
The task: If you're right-handed, use your left hand for daily activities (or vice-versa). Start with brushing your teeth left-handed, and practice until you have perfected it. Then try to build your way up to more complex tasks, such as eating.
The reason: This is an exercise in which you know what you're supposed to achieve, but must do it in a new and demanding learning context. Doing such an activity can drive your brain to make positive changes. Think of millions of neurons learning new tricks as you finally establish better control of that other hand!