According the the University of Miami Biology department:
The basic diet of a domestic rabbit should be primarily Grass Hay! This means not alfalfa or clover. Mostly because this hay is richer and higher in calories, protein, and calcium than most rabbits need. If all that you have available is alfalfa or clover hay that is better than no hay.
In order to ensure that your rabbit gets all the nutrients that it needs you should also feed a quality pellet. Most commercial rabbit pellets feature an alfalfa based pellet. You should try to avoid pellets that include Corn fillers. These are not beneficial to your rabbits diet and can harm their system(source see #7). If you look at most of the common rabbit foods on the pet store or supermarket shelves you will find the corn or soy fillers as well as many extras that your rabbit does not need. For this reason I personally recommend going with a quality commercial grade food like Purina, Pen Pals, Manna Pro, or other quality feed. Most areas have a farm supply store near by. I recommend talking with someone there to help you choose the right food. You will probably find that the larger bag of food will be cheaper there than the small bags of filler at the pet stores.
There are a few premium foods like Oxbow Essentials - Adult Rabbit Food or Zupreem that feature a Timothy hay pellet. None of the pet of Farm stores in my area carry them, but I did talk to my Farm Supply store and they were willing to order it in for me.
You do not need the highest quality pellets for most rabbits. If it is just a pet house rabbit then the 14-15% protien is fine, this is usually the cheapest variety. If you have a wool producing rabbit you may want to consider the 18-19% protein blend because this will help with the keep their fur nice.
I limit my treats to 4 different things.
Parsley because actually helps a rabbits sensitive gut maintain balance. Parsley comes in bunches but if you give the rabbit too much you are liable to cause diarrhea. I generally limit my rabbits to 2 or 3 sprigs.
Carrot Greens When we make dinner with carrots we purchase the carrots with the green leafy tops so that we can treat our rabbits with the tops. The mostly love the greens but will also sometimes eat the orange tops.
Dried Papaya We treat with a small piece of dried papaya every few months. Usually for a few days after we notice a little more hair in their poop. This is the desired effect. If you have a wool(angora, Jersey Wooley, Lionhead, or Fuzzy Lop) breed you may want to increase the frequency to monthly. This is a sugary treat so you do not want to over feed it a small piece goes a long way.
Dandelions We pick them greens and flower then rinse them well in cold water. I would never use these if I thought there was a risk of pesticides. But dandelions are a very mild food and are one of the first treats that are safe to give young rabbits. They will eat the flower, stem and the leaves.
There are other things that you can use for treats as well. But it has been my experience that rabbits do best when their diet is constant and consistent. So what ever rabbit safe treats you choose choose a very few types, and stick with those. Never feed Seeds, Nuts, or anything that can cause gas. The rabbit system is fragile and these things can disrupt the balance and often with fatal results.