If I were going to do veggies in a window, I would do lettuce, herbs, and onions. If you have the height necessary for a tall plant, you could try tomatoes or peppers. If you really wanted to get creative you could do carrots (a root veggie). You would have to stay away from anything too tall like corn, or vines like cucumbers. You could grow potatoes in a large pot on a balcony, but potatoes require a lot of digging, so I think they are a lot of work.