Diet should be made up of 80-90% hay and then a few pellets and a small amount of veg.
Pellets mean that the bunny doesn't selectively feed as they would with a mix food.
Veg can be made up of herbs - parsley, coriander, basil etc
Veg can be cabbage, greens, broccoli (small amounts), celery, but avoid giving sweet veg like carrots and corn in large quantities.
In my opinion it's better to give a little of each so that the stomach gets used to the foods, rather than keep giving different things suddenly - rabbit guts don't like rapid change.