Mbuna means "rock dweller" so it is always best to provide plenty of cover where these fish can escape and live. For more natural tanks you will want to provide rocks the length of the tank, simulating the rocky areas that are along most of the shores in Lake Malawi. Any type of rock will work but slate and river rock are both popular. A sandy bottom will give your tank a more complete look, but you only need 1/2" or so, if you use more sand then it is more work to keep it clean. There are not any plants to speak of in Lake Malawi, a few in the sandy areas, but not on the rocks.
Good Luck with your tank!