Discussions of the current debt limit controversy mostly take it for granted that, if the limit is not raised, the federal government will either have to cut spending—often, it is implied, by defaulting on the interest obligations on the present debt—or raise taxes.
There is, however, a third option, one that has gotten quite a lot of attention in the Greek case: Asset sales. The U.S. government, like the Greek government, owns a lot of valuable stuff, most obviously land and buildings. I don't know what the total value is, but it is hard to believe that it isn't substantial. In the Greek case, I have seen it claimed that selling all such assets would come close to liquidating the national debt.
Of course, there might be technical problems to doing it quickly, if only because of legal restrictions on how such can be sold.
Robert Murphy has a fairly detailed discussion of assets that could be sold, concluding that the total of reasonably liquid ones is about $1.6 trillion. That doesn't include buildings that are currently unused or underutilized, and other more difficult to estimate (and perhaps to sell) assets.