The obvious way to make money in Sims Social is to start gardening. Each property comes with four garden plots. (You can buy more when you have enough money and are at a high enough level. They cost $1,000 apiece, and are unlocked when "Maximum Reached." I don't actually know what that means. Maximum gardening skill, I guess?) 

 

Click on the plot and you have the option of planting any of a number of harvestables, from strawberries to grapes. Each one takes a different amount of time until it is ready to be harvested. The longer it takes until it's ready for harvest, the more money you earn for harvesting it.

After you have planted something, you have the option to water it. In the experiments I have run, watering a plant does not affect the amount you earn from it. (Unlike the PC game The Sims 3, in which you must tend your plants, lest they die before harvest time.)

 

The more important consideration is harvesting them promptly. When the plant's harvest time arrives, a big green pointer appears above it, and the plant sparkles. If you harvest it while it is sparkling, you get a special "Freshness Bonus" which means that it earns you more Simoleons. 

 

If you wait too long after harvest time, the plant will wither and die. Not only will you not earn any money from it, but you will have to spend an Energy point to clear it off your plot.

 

The amount you earn per plant is kind of irrelevant. Clearly you could make the most money per time spent just by churning strawberries. A bigger consideration is when you will be returning to the game. If you want to maximize your gameplay, your best strategy might be to plant and harvest strawberries for as long as you're interested in playing that session, then plant some long-term crops to harvest the next time you play - whether it's in an hour, later that night, or tomorrow at the same time.

 

The game spends a lot of effort encouraging you to return on a regular basis. I would recommend checking in at least once a day. You get a special bonus for every day you log in, and an extra-special bonus if you fire up the game five days in a row. Thus if you can get on a routine of playing at the same time every day, you can harvest grapes and maximize your gardening time.