You have to be careful about that sentence. The only other options is to sell $ for another currency.

Anything that they buy things that are denominated in $ are usually US imports hence goes back to the net export calculation.

Of course they can also use to buy global commodities like Gold, Oil which are denominated in $$, but that is just a proxy for selling $ in foreign exchange and using that currency.

So, logically only 2 options. Sell US $ in FX or buy US assets (Bonds, Stocks, Real Estate)