Stillin1 said - "With anyone on their first flight - just to see the look in thier eyes when they finally understand why you must do this."
Done that.

Took my seven year old son on one of the Classic Wings Rapides at Duxford this summer, and just sat watching his face as we took off. He was looking out of the window at the world rushing by, when all of a sudden, the ground fell away... he turned to me, all wide eyed and grinning, pointed out of the window, and roared with laughter. He understands.