French Fry Stand
I worked concessions at various Midwest fairs as a summer job for about 10 years. The company I worked for had been doing it for 60 years and had about 40 French fry and lemonade stands.
The margins were insane. Pay kids minimum wage all summer. There were multiple times each summer where my entire cash draw was being lifted up with $20s, $50s, and $100s that were kept under the draw.
$4-$7 for 50lbs of potatoes that turned in to $700-$1200. We would go through 10-30 bags of potatoes per day per stand.
Hardest part about that industry is all the good spots at fairs are taken by the people that have been doing it for decades. Once you have a spot, the fairs let you keep it until you get kicked out for doing something dumb or retire.