My classic one is trying to convince people to make grilled cheese sandwiches with mayonnaise (on the outside). Lots of people don't believe me that it's any good, lots recoil with disgust, and every single person I have convinced to actually try it has converted to making grilled cheese this way.
Basically, instead of butter, you slap a decent (but not insane) layer of mayo on the outside of your bread. Then you do the grilled cheese thing as normal. Works on soft white breads, on fancy baguette grilled cheeses, and lots of others. Yields a profoundly crispy greasy crunch, whereas butter can sometimes give you a softish limp crunch/toast (depending on heat and whatnot).
Mayo for grilled cheeses. Just try it ONCE.