Emotions were the safest of all, especially if I avoided saying just who was feeling the emotions in question. The book is written such that a multitude of emotions are cycled through on a regular basis, so it was fully safe and entirely non-spoilery to say things like "Someone's bored now." and later, "They're not bored any more."
Furthermore, I could safely reveal how the book ended - we'd been wondering for a while if it would be a full stop, a question mark, an exclamation point, or - if it ended with "The End" - no punctuation at all. The answer? It ends with a full stop (i.e. a period).
P.S. Please don't be spoilery if you leave comments on this post.