IMO nothing beats gardening when it comes to income!
I have 8 perfect UFO plants at my current household and my Sims get anywhere between $9600 to over 18k every 3 days or so and they don't have to water the plants. My Sims get the higher amount if they weed, spray for bugs and fertilize when needed. The lower amount of $120 per fruit is from an unattended plant.
Whenever I start a new household I just send the new household over to my master garden that has all perfect plants and have them harvest it. One thing I do not like about TS4 is the repetition and this takes care of all the evolving plants etc. For the aspiration I just have the gardeners form the new household gather local plants to accomplish the goals and after that I store most of the plants in the household inventory once they have the refrigerator well stocked. I can pull plants out of the inventory so my Sims can restock the fridge whenever the fridge gets low.
