Here's one answer: He could have told John Ashcroft to stop cutting the FBI's counterterrorism budget!
But that's too short for a diary. So my larger point--I'm worried that we're going to lose an argument that overreaches, and says Bush could have stopped 9/11. That seems to be where the argument is heading. That, or vague sort of "he should have done what he did (tell the FBI/CIA to work on it), but more vigorously."
I think we're better off focusing on some key specific actions that show Bush mismanaged security. We need to back off on saying he could have stopped 9/11 (a meme that is just undigestible by too many, and speculative anyway), and instead focus on examples that will add to the general meme of no judgment/no management abilities.
Got any others?