Just a massive number of cheap crops, depending on the context of when/how soon you're trying to up your farming levels, you can even use super cheap and dirty crops and leave them out to get watered by the rain.
Wheat over the summer/fall is the prime candidate, as wheat seeds only cost 10g per, need only 4 days of watering, are harvestable with a scythe, and can be processed into 400g beer (assuming you have the level 10 artisan skill, which you totally will after harvesting all that wheat) using a keg, if you decide to get some basic processing infrastructure up and running.