Buy Treasury?

The U.S. government can be considered as the most reliable borrower in the world because it has never defaulted on a loan, and it would take a mighty big catastrophe before the U.S. Treasury could collapse. To put it simply, you'll never have to worry about the U.S. not paying you back if you buy some of its bonds.

And of the various different kinds of bond the United States government issues, the Treasury Bills have the shortest maturities and hence the safest.

• Treasury Bills have maturities of one year or less.
• Treasury Notes have maturities of two to ten years.
• Treasury Bonds have maturities greater than ten years.

(The fed is privately owned)

If the us goverment fails to pay a bill is because civilization is collapsing and by such, money is irrelevant and by such you won't care about losing your bond. Also the us goverment can print money anytime they want without much repercussion in inflation as US dollars are highly demanded by pretty much every person in the world, so the market happily absorb the extra printed dollard. The us would never default a loan as it would be a immense sign of weakness to other countries.