Sovereign debt of a more politically stable nation state or other monetary union, if you are investing at these levels. If you're an individual, you have more options, although there will be fierce debate about the risk profile (as US Treasuries were historically considered to be risk free).
Which nation states might you consider more politically stable than the US, even now?
But I am having a hard time identifying this union/nation. Unfortunately, it feels like the EU is set on a downward trajectory.
The disciple went to his master and said "Master, I am considering stopping doing a thing and starting to do a different thing. But I am not certain what the new thing is that I should be doing".
The master turned to the disciple and said: "A better thing"
The disciple was enlightened.
EDIT: Oh damn it. The entirety of the comment was "Sovereign debt of a more politically stable nation state or other monetary union" at the time I replied. Ah well.