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.
American cheese. Your cheese would gain value over time, because the held amount would diminish in the form of grilled cheese sandwiches!
Pretty sure that’s how that works.