For rabbits less than 3 months of age, it is best to keep them on a diet of strictly feed, as their digestive systems are still forming, and if you try to give them too many other things, they will probably get sick.
Rabbits cannot vomit, therefore, they will often times die from digestive problems very quickly.
After 3 months you can introduce hay, timothy, carrots, apples, dandilions.
Make sure you are still feeding pellets to ensure the constant diet will allow for less tummy upset. NEVER allow your rabbit to roam your yard and nibble on things, as I have known rabbits to eat toxic things, or accidentally get into something it shouldn't. As long as you keep your rabbit well fed, it's cage clean, and have plenty of fresh water, it should be happy and healthy.