What is a good, quick way of making money(bells)?

I'm not sure if this qualifies as "quick," but the best way I've found to make money is to plant an absurd amount of foreign fruit trees in your town. That is, any fruit that isn't native to your town, whether it's apple, orange, cherry, peach, or pear.

Anyway, each tree will grow three fruit, each fruit being worth 500 bells. So a tree is worth 1500 bells and if you pick the fruit, it will appear again three days later. I've got enough trees in my town to make about 120,000 bells every third day, though the harvest takes about an hour.