Avoid corn- it can have a mold that causes neurological problems, which ultimately causes dead bunnies.
I feed mine barley, oats, sometimes wheat, and BOSS (Black Oil Sunflower Seeds). I also add beet pulp shreds to the mix. They get alfalfa hay and whatever greens are available at the time.
If you go pelletless, they need a mineral/salt block- they are red. I buy the 50 pounders that are for horses. Some people use the ones meant for goats (smaller), or you can buy the red spools made for rabbits. The bigger unit you buy, the less expensive it gets. You can either break chunks off with a hammer or saw slices off.