Gardening is one of the easiest ways to make money quickly...and passively. I just started my legacy challenge, and I'm thinking about building up a garden once I get a bit of money. There are a bunch of plants that spawn in the world that are worth a lot (blackberries, snapdragons) and you can hire a gardener to take care of them. When I started playing seasons on a different save that I have I had a few of the gardening boxes, scheduled a gardener every day, and all my sim would have to do was harvest. It made me around 10K a day with no effort. Pets can also make you a lot of money if you live in that world. Cats bring you feathers, which again is 0 effort for your sim, and they're worth a ton.
Good luck!