A rabbit's diet should be made up of good quality pellets, fresh hay (alfalfa, timothy or oat), water and fresh vegetables. Anything beyond that is a "treat" and should be given in limited quantities.
When shopping for vegetables , look for a selection of different veggies--look for both dark leafy veggies and root vegetables, and try to get different colors. Stay away from beans and rhubarb.
Hay is essential to a rabbit's good health, providing roughage which reduces the danger of hairballs and other blockages. Apple tree twigs also provide good roughage