Where are the sidelines in a Treasury default? Treasury bonds? Cash? Gold? Corporate bonds? Euros? Yen? Real estate? Commodities?
What to buy depends on how bad things are. During a very extreme crisis like Russia's 1917 revolution or the rise of Nazi Germany, people favor gold because it's a very dense form of money that is easy to transport, hide, or smuggle. Obscenely rich people often have at least some of their wealth in the form of gold bullion. At gold's current price of $1300/oz, a pound of gold, about the size of a deck of cards, is worth $20,800. Hungarian chemist George de Hevesy dissolved gold Nobel Prize medals in acid to hide the gold from the Nazis. Few things are as safe as gold when it seems like the world is ending.
Some other possibilities are to hold different currencies. Germany experienced hyperinflation in the 1920s, but America's currency was backed by gold. Converting German marks to US dollars at that time was a safe hedge against inflation. Countries with good balance sheets and stable political systems are more likely to have stable currency. Norway, Finland, and Sweden have some of the lowest debt loads in the world, so I would expect their currencies to be some of the safest. A quick google search shows that Norway and Sweden have their own currency, but Finland uses the Euro. I'm not too fond of the Euro because the EU has so many loser countries in it that would rather go bankrupt than cut any social programs. I'm thinking about countries like Greece, Spain, France.
That said, I would probably just sit on the side line in US dollars, but have the cash in an investment account and ready to buy something. Defaulting on national debt is the kind of thing that causes panic selling and market crashes. Wait for the crash then buy large cap companies that operate on a global scale. Coca Cola is sold in every country. McDonalds is in every country. Intel is in every country. Microsoft is in every country. Oil companies like Exxon and Chevron are global.
Bonds are the absolute worst thing to hold during a crisis. Things like the price of gold, real estate, and stocks automatically scale with inflation, but bonds move in the opposite direction. The whole point of having a positive inflation target is to welch on bond payments. If a bond is set for 5% interest and the inflation rate is 8% due to wild money printing, the bond effectively has a negative interest rate. Adding insult to injury, the value of a bond goes down when interest rates go up. Your crappy bond is paying 5% but the newly issued bonds are paying 10%. The only way to sell that crappy bond is at a steep discount.