Here are some that we've been to:

1.  Hays Dairy Farm - Off Lim Chu Kang Rd at Lane 4 Website: http://www.haydairies.com.sg/
To get there: Bus no. 175 from Choa Chu Kang interchange. Alight at Lim Chu Kang Lane 4.
Visit early in the morning by 9 or 10am to see them milking the goats. This two-hectare farm has about 500 goats and produces some 250 litres  of milk a day. Goats' milk has a natural hazelnut flavour and is richer in Vitamin B and mineral salts, and the fat globules are smaller than that of cows'  milk, thus making it more easily digestible. It is thus said to be good for asthmatics, youthful skin, allergies and people with lactose intolerance.  The children would love watching the kids (baby goats), learning about and identifying the different types of goats.  You can taste & buy fresh chilled goats milk at the farm.

2.  Quail's Farm, Lian Wah Hing - 5 Lim Chu Kang Rd at Lane 6F
Do not visit if you've been to the zoo or come into contact with any chicken or birds the day before as you may carry diseases that affect their fowl.  There's turkey, ducks, chickens, and of course quails in this farm.   You will get to sample some cooked quail's eggs and can buy them cheap & fresh from the farm too.  You can also buy quails & herbs to bring home and boil herbal soups and tonics.  Quite pleasant-smelling and tasting!   If you visit on a Sunday, you can even taste the different types of herbal soups that they have for guests!

3.  Jurong Frog Farm - 56 Lim Chu Kang Rd at Lane 6 Website: http://www.jurongfrogfarm.com.sg/
Bull Frogs, tiny frogs (don't know the name, but they tend to feed these to Arowana fish?), tadpoles, etc. These water bullfrogs are known to many usually as delicacies. Besides frogs, there are also eels, turtles, tilapias and snakeheads in the farm.

4. Aerogreen Technology - 115 Neo Tiew Crescent, next to Sungei Buloh Nature Park.
Singapore's first commercial aeroponics farm.  This farm which uses aeroponic technology to grow vegetables.   Aeroponics, which means growing in air as opposed to organic (soil), enables the growth of temperate vegetables in equatorial Singapore. Nutrients are sprayed directly onto the suspended roots of the plants and vegetables.   Many types of vegetables are grown here namely Butterhead Lettuce, Batavia, Baby Cos, Kai Lan, Bai Cai, Lollo Rossa and Japanese Yukari (Chye Sim) as well as cherry tomotoes. The Butterhead Lettuce is fresh, crunchy, sweet and juicy. The farm produces some 500 kilogrammes of pesticide-free vegetables a day for local consumption.  Sometimes, you may even be in the season for strawberries!  Go on Saturdays and you can do harvesting.  All the vegetables you can pluck for your bag for one good price!  You can also buy their pre-packed salad bowls, lettucino drinks and jelly. Contact them to arrange for visits

And after you are done with the Aerogreen Farm, you might just want to pop over to the Sungei Buloh Nature Park.  Check it out!