We do that, accumulated fresh veggies from several family and friend's small "garden" plots. All home grown and as organic and fresh as we can. Delivered and/or collected at the house for cleaning and sorting in the pre-dawn. Then given to another relative to sell on her market stall. Quite lucrative in the cool season (right now) but forget about it come March when it's heating up and bugs need serious pest control. Very (short) seasonal income. Scaling up would probably mean investing in land, pesticides, etc..
Another source of seasonal or opportunistic income is from foraging. Depending on the season, frogs, waterbugs, ants, land crabs, snails, mushrooms and other short-lived but quite expensive flora and fauna makes its way to the relative's market stall where some city slickers will pay (almost) a king's ransom for some weeds, grubs or furry critters. Keeping an eye open for a natural bee hive full of wild honey is a tidy little earner with some happy to pay upwards of 400 baht for a large Saeng Som sized bottle of the stuff.
The other one that keeps us in beer and skittles is the roadside food stand that, as already suggested, specialises in something that's NOT already offered nearby. No need for somtam wars in the moobahn! Minimal dine-in as it caters for locals either going to work in the morning (closed after lunch and rest of the day) or, the evening commuter headed home. The latter is a bit less busy with a greater possibility of a dine-in request. You may also need to offer more than one specialty dish too (open late afternoon and evening only).
Thai landlords are as mercenary as the ones back home so if any chosen enterprise avoids paying rents, leases and inflated utility bills, it's going to be more profitable (take longer to die).
Good luck