Something that can easily be stacked onto SPIN plots like you talk about are rabbits. They don't take up that much space, they love eating grass, leafy greens, weeds, & other plants people usually don't consume and their guts act as a quick composter to process the waste material from the veggie / produce farm into fertilizer on-site. Plus they are quiet which makes it possible to raise them in urban environments. I am currently only raising them (as well as my garden plots) for presonal / family consumption, but it wouldn't be that difficult to scale the operation up to earn some $$$ from them.