I opened a martial arts club and turned a hobby into a revenue stream. I did teach a club before but it was for little to no income, i look back now and consider it my apprenticeship because I am good at what I do. I know this because when I left that club I was getting phone calls for weeks from parents begging me to come back, the club went from 40-50 students to less than 15 with the new instructor.
I now have 40 ish children and 15-20 adults each week and am contemplating a second club with a view to a possible 3rd and then creating an association but that's only a possibility and not a concrete decision.
The main reason I went down this line is that not everyone can simply open up a martial arts club. I live in a rural area (very rural!) and there is nothing else out there for the younger kids in the way of activities so I do have a monopoly and the parents are glad to have someone to offload their offspring to for just over an hour.
I only have hall rent and insurance to pay, I bought all the equipment and was paid back from club funds within a month.
Good business IF you're good at it and prepared to put up with little children