In all my game play, I found that:
- Gardening gives you a lot of money.
Literally my sims quit her job cause her gardening alone gave her more than thousand simoleons each time she would take the products and sell them.
- Writing books also gives you a good amount of money as long as you write regularly because the money you gain decrease with time.
- You can do painting for a living, in the painting career or by yourself, and you will have plenty of money after she's trained a little bit (also works for books, the value increase when you get better at writing, but the beginning is a little rough)
- Finally, the tech career is the career that pays the most (even at the beginning), so it is the easiest way in my opinion to win a good amount of money regularly, without having to constantly do gardening, painting, or writing.
Alternative way to win money is also looking into boxes on the street and sell what you get (if you have city living) or repairing stuff and selling what you get. But the prices would never be really high I suppose so that makes it hard to live off just that
