I have never understood why terrorism always required the death of many people. Look at 9/11, the damage caused was huge, a huge number of deaths as well as a big economic damage. Which is good for a terrorist attack, but the work they had to put into that must have been so huge.
Instead I would think that "smaller" economical targeted attacks could be a lot better. Consider for example a GPS/GSM jammer. These would be extremely cheap to make compared to the whole 9/11. You can easily jam a big city with one home made jammer. You can easily put them up somewhere without any cameras or security and still cover everything you wish to cover.
Just imagine the economic damage and chaos coming from it. All GPS stopping means almost all roads will have major traffic jams as well as any delivery service will get totally fucked. Even better, with added GSM jamming, nobody could even call somebody for help. It will take hours for police to get prepared for anything.
The major downside is that jammers are very easy to find. You look where the signal is strongest and kill that. Still, I doubt most communities are prepared for a total wireless blackout. And you could set up hundreds of jammers on a relatively small budget without danger of being found out.