There are several, so it depends on where you're going to be. The best place is Tropical Plantation north of Hato Mayor, which is a working flower farm (three kinds, canopy, tree and hothouse farms), which also run you through their several-hectare grounds showing you the various endemic trees to be found, along with birdlife, a typical campesino farm, plus examples of dozens of common crops as they are grown.
In the northeastern coast town of Cabrera, a hotel called La Catalina runs weekly tours to a working coffee plantation called Chez Jose that are quite good. American Express has a four-day tobacco tour that runs to some tobacco farms; otherwise you can drive to Tamboril and check the farms out for yourself. In Jarabacoa, Rancho Baiguate runs culture tours that go to a coffee factory and a cassava bread factory. There are also cassava factories in Moncion that are interesting, but this is a very out of the way place. In La Romana at the Jardin Dominicus restaurant, you can sign on for a boat/jeep tour that will take you to a peasant farm and a tobacco plantation. It's also worth checking out on your own the Haitian bateyes, where Haitian braceros chop fields of cane and carry the stuff back on oxen-powered carts. I'm sure that there are others, as well.
