My better half can sleep sitting up. So when we have to get somewhere I am the designated one to keep the driver awake. My favorite tactic is something cold placed gently and precisely on the face and/or neck. Repeatedly.
I have found from experience that the one thing that is certain to keep you awake far longer than anything else is almost getting into an accident because you were drowsy.
Matter of fact, last time I did that nearly forty years ago, it still keeps me awake when driving today.