well, technically any solute added to water will depress the freezing point (however, the vast majority of those solute compounds are def not those you want cats ingesting).. salt is effective and used often on roadways and such because it's cheap, but sugar, like suggested above, also lowers the freezing point, but it's still not particularly dramatic (let's say, 5 degrees F, I'm not sure exactly at all off the top of my head). However, a significant amount of sugar would have to be added (~1:4 sugar to water ratio) to have the full effect. I'm not sure if sugar water like that would be healthy for cats though (for teeth though, maybe leave out those treats that are supposed to promote good dental health??). But it's probably the only readily available, non-toxic, material.
At my house we have a heated birdbath, maybe you can find one that doesn't look fancy, and just looks normal, or maybe could feed them by your house?