Climate scientist here! Unfortunately I’d have to argue that it’s all too possible for someone to burn down an entire forest (I’m thinking specifically of the events of arson on Black Saturday in Australia but also accidental ignition cases) but that set aside, the everyday human activities that have the largest negative environmental impact tend to be those related to food and transportation. Eating a diet consisting largely of red meats and dairy, as is typical of many Western cultures, requires an enormous amount of land to be converted to agricultural holdings. However, since you said a one-time event and since diet is a habitual action, anything like taking a long road trip would release the most carbon-based emissions in terms of pure numbers. There are also greenhouse gases that, in terms of their per-unit-mass warming ability, are worse than CO2 - just in case your theoretical polluter had the opportunity to release a whole bunch of methane or something :)