grass

remember you can't eat grass direct,
the cellulose is not digestible by humans (don't try to just go eat it)
but you can juice the grass and drink the juice.
remember that there is food all over if you go looking for it, it may not be great food, but it sure is something

the more tasty way is to get animals to eat it, then you drink the milk or eat the animal,
these don't eat grass so they are of no use that way...