Worm farms are easy to set up and take very little effort to maintain. The compost (worm castings) produced by the worms is a rich fertiliser, excellent for nutrient-hungry plants like tomatoes and roses.
They can be placed on a balcony - all you need is a small, cool, well-shaded spot.
Worms will happily live a vegan lifestyle. Worms do eat meat but it is best NOT to put meat in the worm farm as it will attract flies. They love rotten fruit such as apples, bananas, peaches, avocados but avoid citrus fruit as it's too acidic. Vegetables are devoured but avoid onion, garlic and other spicy vegetables. Coffee grinds, tea leaves, paper and other organic matter is great, but never feed them dairy products or eggs.
The book Organic Gardener's Composting by the Australian Steve Solomon has an excellent section on worm composting (vermicomposting). Solomon's book is also available in Dutch: Composteren voor Biologische Tuiniers. Should these links go stale, Soil and Health Library is Solomon's repository of texts.