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.