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.
On a more serious note, it should be something no single country has a majority of control over, and it shouldn’t be a limited resource, like gold which was previously used.
Trees? There I fixed global warming.
If every dollar printed meant the US planted a tree, then anytime a tree is cut down (even on private land) you have to turn in a dollar to be taken out of circulation, it technically would work.