You have to put in the factor of your labor. While you don't pay yourself to do this, the time you spend taking care of your crops is a time you could be spending making money some other way. Therefore besides the opium(reason is I don't exactly know if it's legal where you are) from the other options I think potatoes are the best.

They need little care, you plant them easily, then they're underground and you don't have to worry about a hailstorm for example the way you would if you plant say nectarines. Potatoes are survivors, you might have to water them sometimes if there's a drought, but all in all you just bury them and come back a few months later to collect.

Bonus is they are a long lasting product and you don't have to worry about them going bad one week after you took them out of the ground.