[mikeash]

Dry ice will, ironically, make your soda go completely flat as it constantly agitates the drink as it boils/sublimes away.

[Groxx]

but it'll make water pretty carbonated (though not to soda's level). or at least have the same bitter, bubbly flavor (never really looked up if that's actually what happens)

[nealabq]

Cubes of frozen alcohol might work. Maybe with a little frozen C02 mixed in for certain drinks.

[mikeash]

I love it! Now I just have to figure out how to generate a temperature cold enough to do it.