An example could be holding the US ambassador to China, as well as American tourists hostage until the US turned over the pilots responsible. More extreme routes could be sinking a US navy ship or shooting down a US airliner as an act of revenge.
Could it trigger a second cold war? How far could they go without triggering a full-out nuclear war? I feel they could go pretty far, perhaps as far as a massive terror attack on US soil as the American government would be too scared of a nuclear apocalypse scenario to really push the button when it came down to it. They'd be more likely to use extreme sanctions and diplomatic pressure as their response in my opinion since the fear of nuclear war would remain too strong even in the face of some sort of horrible attack.