If you want to go out with a bang, use a truck!
Somehow Homeland Defense has still managed to turn a blind eye to closing off the means of the first WTC bombing and McVeigh's outrage in Oklahoma.
You can still rent a truck or van, load it up with a bomb and detonate at the location of your choice.
Just show a drivers license and credit card -- no passport or background check or special visa required.
Interesting that the authorities can impose all sorts of hoops and security checks on aviation while drivers get a free pass.
A GA a/c flown into the side of a building can't do much more than rearrange some furniture, as we saw in Tampa.
Many years back an MU-2 trying to land at Edmonton Municipal flew into the side of the local hospital. Amazingly the crew survived and were treated on site.