I realize that you're being somewhat facetious, but don't think of the hobby itself as the income generator. Think of something that is difficult or expensive, or time intensive about working on your car, and try to make it easier, cheaper and or faster to accomplish. Think of it from the perspective of people that work on cars for a living. Anything that saves them time/labor/equipment costs will be of interest to these people.

The bare essence is this: find a problem that a large group of people experience, and find a way that allows people to solve that problem.

Mechanics are people who enjoy working on cars; and beyond that, cars provide lots of space to tinker --- I remember being inspired by old Mother Earth News articles on converting your pickup to be wood-burning, or vegetable-oil-burning, or any sort of alternative fuel. Farming/gardening is a great way to play with your kids, and make a little money on the side too; the money is likely to not be significant, but the pleasure at making money from the work of your hands is personally palpable, since I normally am a knowledge-worker.