Going to a farm is one of the more interesting things you can do in Japan. There are many reasons to go to a farm, as there are many different types.
Fruit Farms - Japan has some of the best quality fruit in the world. The farmers put a lot of time and effort into making the fruit as delicious as possible. There are many fruit farms in Japan, and you can pay a certain fee to pick/eat them at the farm. The most popular would be the strawberry farms. But there are also other activities to do such as digging up bamboo shoots and picking mushrooms. Check it out.
Animal Farms - for the animal lovers
Flower Farms - beautiful flowers
Dairy Farm - not many things to do, but you can sometimes find a local farm that sells some delicous milk/cheese products
Rice Paddy Farms - sometimes you can find events/festivals where you can partake in planting rice (called Taue) It usually takes place around the end of May. So if you`re interested, check it out!