Just buy an icecube tray. :P

You can try putting coke in it instead of water. Then just add those coke-cubes to your refrigerated coke to cool it off without watering it down (yuck).

Or, if you have plastic bottles, open one and pour a little bit of the coke out and squeeze the bottle until the coke-level inside is just below the overflowing out the neck. Then you can freeze it and the plastic will have more lee-way to expand when the soda freezes and expands.
-If that didn't make sense, it doesn't matter. You can't even drink solid coke from a bottle anyway. :P