Its not really that easy to get ak-47s into a football stadium.
9/11 was a unique terrorist attack that used the resources inherent in a plan as the main explosive force. To get those type of results you need a lot of fuel and that just takes up a lot of space. The biggest threat remains a OKC bombing style where a truck/etc is used with fuel to create the explosion. You could possibly ram it into a stadium etc, to get maximum effect. Other possible options would be train, airplane etc.
The blimp would be hard simply because of access. Regular people dont ride in the blimp to take over the cockpit. Im not even sure if they offer blimp pilot training.
At the end of the day it takes a lot of resources to pull these things off, terrorism is a bad business model.