I remember a story of someone shooting industrial transformers. One bullet from a sniper rifle, and the whole substation is down, millions of dollars in damage.

Two people can rent a couple of UHaul trucks and block almost any highway. It will take hours to get the trucks away.

I also remember an estimate that ~10000 trained saboteurs can bring the whole country to a grind. It's quite cheap and fast to blow stuff up, and much more expensive and slow to restore it.