Oats have a lot of protein per calorie, and they're easy to make. Many cats like corn, and polenta, a coarsely ground cornmeal, has a good texture for them. You can try brown rice, barley, and wheat berries, but you may need to mash them first. Cats tend to like smaller grains like millet and couscous. Just make sure any grains you give are cooked so your kitty can digest them fully. Whole wheat breadcrumbs are OK, too.