have a look at the page here- http://www.greenigsociety.org/foodchart.htm.
Generally lettuce is no good for them. It has little nutrition, look for collard, mustard and turnip greens.
The pellets are ok for emergency but even then I would try to avoid it. If you do feed them, soak them in water,since dehydration is such a problem , they get most of their water from what they eat.
A nice colorful salad with squash,green peppers, green beans, sweet potato and a few berries are all you need, course you can add other veggies and change it for variety- that food chart link is really helpful.