I usedto do fruit harvests. But then I risked all my money, a few hundred thousand, on turnips. I bought however many of them for 70 bells, I think. And sold them for 430 bells. Estimated amounts.
So I became quite rich.
I sell pears. That is, until the trees run out of pears. Then I sell whatever furniture I don't want, and if I get any gifts from any of the residents that I don't like (which happens a lot), then I sell those too. I occasionally use the stalk market, but I don't invest unless I can buy the turnips for around 60 bells, give or take.
I also put 1000 bells in the bank but I can't take those out until I've waited for a long time so it can rack up interest.