Well I'm not Zeb, but here's my two-cents on this.
Green vegetables are outstanding - Broccoli, Spinach (especially), greens, asparagus, etc. Raw is better, steamed is ok. Drenched in butter or cheese is none too useful, but still better than deep-fried doughnuts.
Quantity - hard to overeat (but probably not impossible). How about aiming to get all your meal carbs from green veggies (excluding post-workout)? That would be a lot of veggies, but wouldn't be too much.
One tip: chew, chew and chew if you want the full benefit.
If you want to find out why veggies are so healthful, do a search online for body pH - veggies give an alkaline response in the body, which has tremendous health benefits. Some "experts" argue that an alkaline body cannot get sick. That seems a bit of s stretch to me: I don't necessarily subscribe to that theory, but I do agree with the trend.
There's the usual antioxidants, vitamins, fibre etc. benefits too of course.