FWIW I suspect that a few "dirty bombs" may be quite spectacular enough. Imagine a few Oklahoma City type bombs with a significant amount of radioactive material .... It could be nasty even without a true nuclear bomb - especially since a lot of people are irrational when it comes to anything radioactive.
The simple fact remains that UBL knew he was going to get routed in Afghanistan after the attacks... he had to have plans ready to go.
Possibly. Remember that they had just gotten done with trouncing the Russians, who were much closer and should have been able to project a lot of force into the area, and they still got their clocks cleaned in Afganistan. But he had to realize that we might do better, especially since we had a chance to learn from their experience.
Even without nuclear weapons, there are still quite a few things they might have prepared that could cause significant disruption - even on a 9/11 scale. "Dirty bombs" could easily be part of that, as could chemical and biological weapons. I suspect the first two would be more likely than the latter - not because it's so difficult to put together crude biological weapons, but because anything infectious would have a high probability of "blowback," and because of the technical difficulties of using an agent like anthrax and getting more than a few casualties in the immediate area. (The key to a successful large-scale anthrax attack would be wide dispersal of aerosolized spores - a rather tricky proposition since these are somewhat contradictory goals).
However "dirty bombs" and chemical weapons are quite capable of creating a major crisis in their own right. I'd suspect both of those before either true nukes or biologicals.
I consider coordinated vehicle bombings at heavly populated targets (malls, disney land, etc) to fit the spectacular category, especially if 6 of more. Smaller Bali-type operations (shall we add the recent OK 'incident') would be showy and hard to defend against, but would require more than a dozen to be spectacular IMHO. Less than that would show weakness by the terrorists in that they cannot hit the US with strength, like 911. This has been their tactic of choice, with 911 being an outlier to their MO.