I don't think there is anything strange about the sideline now. All sideline businesses exist because people can get a certain source of income. Therefore, he needs to be able to exist aboveboard. If it is an unreasonable or even illegal sideline, it will not last long. Therefore, some part-time jobs that we can contact or learn in our daily life are relatively formal.
For example, some earn extra income by physical strength, such as distributing leaflets on the street, or doing some service industries, helping people meet customers, or cleaning. You can also choose some part-time jobs at home, or use your brain directly, for example, helping people write articles or codes and help people brush bills, which is easier than manual labor.
But as far as sideline business is concerned, I think the main thing is to find something that suits you. After all, my main business can't be discarded, and my sideline business can only develop after my main business, so I can't put the cart before the horse, and I lost my watermelon and picked up sesame seeds.