A superb spot, inconspicuous, but sensitive enough to cause wincing, squirming, difficulty with restaining her/your body from jumping out of your seat...

and if you're sensatized to it, a euphoria thereafter...

I recommend the Hoku Point.

It is the ganglion located in the webbing, of either hand, between thumb and forefinger, just behind the first tendon/muscle. Squeeze it gently and with increasing pressure causes intense pain with little or no damage.

And just as an fyi, it cures most (non-sinus, non migraine) headaches. 20 seconds.

So the beauty of it is you look like you're just holding hands.