"Can God make a burrito so spicy, even He can't eat it?"
My personal favorite:
Nothing is better than God
A ham sandwich is better than nothing
Therefore: A ham sandwich is better than God.
And that's why they call it Wonderbread!
There are many things that God can't do. God cannot act in an un-godlike way. God cannot do things that go against His nature. God cannot become non-god. I don't know if you were serious, but that is the kind of sophomoric question that is often raised by high school or college students.