How do I punctuate a question with a guessed answer in it?

How to punctuate a question directly followed by the asker's guess at the answer? E.g.

1. What's in here? Your books?
2. What's in here, your books?
3. What's in here; your books?

Which of these (if any) is correct?

I would use What's in here? Your books? when you're asking a sincere question and then sincerely guessing at the answer.

What's in here, bricks? might be used if you're being sarcastic about a possible answer.

And I would never use a semicolon in that example.

You can go with a question mark (1), a comma (2), or an em dash (not a hyphen). A semicolon (3) is not an option.