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Yes corn or sorghum would work, plant it in an X pattern so the birds don't get use to running the straight rows, In smaller pens or flocks cut christmas trees will work also, or depending on the situation if it is difficult to grow things artifical structures such as lean-tos will work. But having planted cover will condition the birds better to what they will see when released.

We simply take a grass seed spreader on the back of an ATV into the pen. One bag of corn seed is enough for a 150x150 ft pen. More is okay, but not necessary--nor do you want to put too much in. The seed spreader throws the corn everywhere, eliminating rows, which is a plus. Corn provides fine cover and provides a great food for fully mature pheasants in the Winter. Corn is somewhere between 6-10% Protein which is okay for fully mature pheasants. But it also helps birds fatten up--which is great for cold areas and impressing anyone who looks at your birds.

Some places have used Sunflower too. I can't speak on that though--I don't have experience with it.

Sometimes just letting whatever weeds grow up can work too. Not something you want to do if you can avoid it. Typically the only reason to allow whatever to grow up is a lack of time to plant due to moisture, etc.