Dandelions are the very best but can be hard to find...
Red or Green leaf lettuce, collards, turnip and mustard greens (in limited quantities), herbs including dill and watercress, endive, escarole, bibb lettuce, radicchio (expensive!!!), bok choy (they love it), spinach, limited kale (they love that too). The dark outer cabbage leaves only... Basically, as long as the veg is dark green they should be able to have it...no beans or peas!