Hmmm, I assume the usual hamburger/hotdog isn't what you're looking for.
I think the least common thing I put ketchup on is kraft mac n cheese. Don't do it every time, but every now and then, when I'm having the standard, package only mac n cheese, I'll put a dab on there.
Can't use much, or it sucks. But the kind of amount you might drop as you're taking a bite of something else, that works great. That's how I found out I liked it, back as a kid.
I will put ketchup on mac & cheese, of any kind, no matter how old your grandmother's recipe is. did it right in front of my Italian potential mother-in-law too, there are certain things that have to be established early in the relationship.
I've upgraded from ketchup on my mac and cheese (we call it Kraft Dinner here in Canada) to using Bullseye barbeque sauce. Adds a little bit more flavour.