The On Nut fresh market is not as expansive and inexpensive as the Udom Suk market a few stops down the BTS skytrain, and it's a bit of a walk from the On Nut BTS station, but it's a great place to check out if you're in the area. It has stalls upon stalls of fruits, vegetables, meat (more organs than I ever wanted to see) and even big buckets of still-alive fish, water-snakes of some kind, and even toads (or maybe frogs?). There is also a nice food court area with cheap Chinese or Thai meals at under $2 each.
Culture and experiences
On Nut Watch
On Nut wet market is not as big as the Udom Suk market, but it's still great.