Ask HN: Lucrative small business ideas?

In the thread "Cleaning restaurant floor for $1000 per night" (https://news.ycombinator.com/item?id=18920438), I have came across opinions like the following: "There's tons of businesses that "print money" as one or two man LLCs but they don't scale well beyond that point."

I thought it would be cool to collect ideas here. What such ideas do you know of? Even if it's hard to scale.