One of the things I got for Christmas was a set of whiskey stones. These are simply cut stones, made here in Vermont, of a size similar to an ice cube, and intended for chilling whiskey.
I don't drink whiskey, or alcohol in general, but I thought they'd be useful for the beverages I do drink: mostly things like soda and lemonade. I am perpetually seeking a perfect solution to keeping these cold. I drink a lot, and I like to bring a pretty big glass of whatever to my desk so I'm not up getting refills every twenty minutes. But I like my drinks cold.
Ice cubes are good for making a drink cold, but not for keeping it that way for any length of time. They melt which not only means they aren't helping keep it cold anymore, they're diluting the drink. I used to use glasses that had a layer of ice inside the glass itself, but while that didn't dilute the drink when it melted, it still melted fast, and then wasn't keeping the drink cold anymore. Making ice cubes out of whatever beverage you're drinking is also good for avoiding dilution, but it still doesn't have a very lasting effect, and if you're drinking different things it's a pain to keep ice cubes for all of them handy. At work, since I pretty much only drink lemonade, I just have two cups and at any time one cup is about 1/5 full and in the freezer, forming one huge ice cube, which lasts a lot longer at keeping my drink cold; but even that's not quite as long as I'd like, and it wouldn't work well for soda, even if I drank only one flavor at a time.
So far the whiskey stones are a better solution than any of those for soda, but they're still not ideal. They do hold their coldness longer than ice cubes, though still not as long as I'd like (larger stones might work better for that). One of the things that makes them good for whiskey is that they don't make the drink as cold as ice cubes would, but that isn't as good for soda. I wonder if there's anything that's as cold as ice cubes, but longer lasting, and not anything that would dilute the drink. I wonder if there'd be a significant market for such a thing.