Really depends where in the USA you’re going.
Potato chips in a bewildering variety of flavors.
Find an event labeled “Festival” or “Fair(e)”, try fried things that you haven’t had fried before.
Philadelphia: soft pretzels. Not from the chain place but they’re ok in a pinch. Bernie’s in Clifton Heights is worth the detour.
Chicago: hot dogs. There’s good hot dogs in many places. If you make hot dogs an event, though, do it in Chicago if you can.
Kettle corn. Can find good versions in lots of places. If they’re making it right there it’s better.
Funnel cake. It’s a fair or amusement park treat. Don’t order it unless you know they make it right there.