Teaching a teenager to drive. Several bullet points here:
A good way to come to grips with your own mortality.
A good way to realize that certain truths you hold to be self-evident were, in point of fact, quite wrong.
A good way to learn whether you can give clear, concise instruction while screaming in abject terror.
A good way to double-dip on bureaucracy. Go to the DMV twice as often!
Did you know that little triangle of pavement that is formed by a highway and an exit ramp (right before you hit the water barrier) is called a "gore zone?" Neither did I. But what amazed me the most: this kid has spent literally his entire life being chauffeured around, looking out the window, and he hasn't paid any attention at all at the mechanism by which that occurred.
This is going to shave several years off my life. I can tell already.