You mentioned that the content has to be specifically Spiritual in nature, isn't that seeing through a lens? The point of that excercise is to not just see but to perceive how you see and what else there is to see.

When there is nothing that is not Spiritual?

Read up on Right Thinking and Right understanding of the Eightfold Path, have a look at the rest too sometimes later. During your retreat ask yourself some questions but don't take the answers for granted. So one of the questions might be "What makes me Spiritual?" Then ask, with Right Thinking and Right Understanding, "What are the reasons for the answer?"

That's deep introspection if what 'underpins' your Spirituality.