> What can I or you possibly do that can wreak havoc on this scale?

Build a bunch of drones that carry explosives and leave them outside chemical plants, refineries, and nuclear plants across the world and then launch a simultaneous attack on infrastructure.

Budget $1000-$2000 per drone and then whatever your costs of transportation for the road trip are.

You'd be flagged and arrested/killed wayyy before you manage to even organize such an attack.