I have put a great deal of thought into this. Everyone is looking for a biological, chemical, or nuclear attack of some sort. I think they should be looking for more of a conventional attack.
Many construction sites have dynamite and blasting caps stored in locked sheds. This material is easily acquired. It is then easy to build a "suicide-bomber" vest - just add some batteries, a switch, wiring, nails and bolts, etc.
So, IF AQ does anything this weekend, I think the most likely form of attack would be 10-20 suicide bombers walking into Starbucks in various US cities, at around the same time. The psychological impact of such an attack would be immense, and the cost for materials would be practically zero.
I used to think of scenarios for a living. That would be the way I'd proceed if I were Al Qaeda.