I know this hasn't been posted in in a while, I hope it's okay I'm replying now ^__^;
I voted the way I make extra money is by solving the unsolveable, which is true as far as my off beat way of making money goes ... but other ways I make money, though maybe not real hard-core, is by picking up any and all seeds, flowers, stones, gems, bugs, etc. and selling them. Some items like bugs and plants, I'll take to the Elixir shop that came with Supernatural; I found that a Mandrake root that sold for only $12 from my inventory, went for almost $100 via the shop; that was a really nice surprise!
I had my Sims grow fruits and veggies, most were between very nice and outstanding, and one day got almost $20k from goods sold at the shop. Granted, I had a LOT of items in the shop - and as I said, were all very nice or better quality. I have my Sims do lots of gardening strictly for money making purposes. I don't want my Sims to be too rich, too fast. Because then I tend to lose interest in the family. So gardening is a slow-paced way for me to keep my Sims with income, without going overboard. Especially the ones that I use as Stay-At-Home moms, gardening is great for killing time between cooking meals and tending to the todd's.