Most of those targets require trained attackers and demolition materials, precise target information etc. They are all possible, but getting trained attackers, target information and equipment together before D-day increases the odds of mission compromise.
Not saying they won't happen, they probably will at some point.
But two missions come to my mind.
/1/ Anthrax: the full broadside. The 2001 anthrax attack was only shot across our bow with 3 grams of weapons grade anthrax. Supposedly Saddam had tons of this stuff. 30 kilos of anthrax would be 10,000 times as much as the amount which caused the panic in 2001. 30 kilos, mailed via USPS, UPS and FedEx in 100s of leaky packages on one day, would shut down our economy.
/2/ Mega forest fires. This requires only a few drivers with a few hundred road flares or other fire sources. Ten drivers could fan out across the western forests on a single day this summer and lead to a holocaust. These fires could be orders of magnitude greater than last summer's SoCal fires, and would totally overwhelm our ability to fight them. Entire towns would be wiped out, with populations cut off and trapped and burned. The health consequences from the smoke and ash from mega fires burning for weeks would be disastrous.
Small planes flying over the forests - well, you know the rest...
You can choose something tactically spectacular or strategically devastating. Smallpox is better saved for Labor Day so the kids can take it to school in the Fall.
You are correct, especially on door #2
I believe they have already been setting forest fires for years.
Lots of prison converts and many fires have been near large urban areas too.