My best (stupid) guess is weapons. US gov has a ton of weapons. Already sells tons of weapons. Now the prices will always remain stable. Other countries would love to know that 30 million usd would always be able to buy an f15. Other countries declaring war will increase the value of the USD, as buying weapons from us government will decrease amount of money in circulation.
Sure taking gold out of a hole to put it in a different hole is a waste of resources but there’s kind of a driving force towards that because if the currency is actually useful then people aren’t going to use it as currency and if it’s easy to produce they’re going to counterfeit it
Same with cryptocurrencies, they’re hard to produce and functionless, so a huge waste of resources, and frankly same with (high frequency) trading of stocks, as you don’t own them when the dividends are paid yet you put huge resources into them because other people want them because of their inherent value
But doing something just for the sake of finances…it doesn’t add anything to the world. The original question was “what do you pick?” and I don’t like the idea of picking an energy-intensive activity that doesn’t help the world in any way.