Probably flying a private and chartered plane over a major metropolitan area and setting off an atomic bomb. That would work fairly well. You wouldn't have to have a crop duster or a big jet airliner. Just get a simple private airplane, one pilot and carry an atomic bomb on it. Set it off as you're flying over whatever significant target you want.
A lot of these Arabs fly their own planes into this country. How hard would it be? What kind of "bagage checks" do they have on their own planes? They can fly straight into this country and do anything they want -- before they land at that "first point of entry" into this country.
Are you going to shut down the entire aviation industry (private and commercial)? I don't think so. Basically, there is no way to protect yourself against this kind of threat (in terms of "guarding against it") -- if someone really wants to do it.
One simply has to eliminate the people who want to do it -- in order to prevent it from happening. But, that's not going to happen -- because the Islamic world basically teaches that they should kill the Jew and Christian (and will ultimately do so -- until they submit to Islam) And -- there are plenty of "Islamics" in the world. This is a battle to the end (of the age...).
The "basic cause" for this has to do with the Islamic teachings. There is an "entire world of Islam" out there, which has been primed -- from children upwards, to follow through with these teachings.
There's the problem.