Well, I know peeps who use fruit juice and alcohol. I wouldn't recommend it since some chemicals can easily bond to alcohol and such, it might reduce the substance with some strains of marijuana. Thus getting you less high.
And of course "depending on the substance" you put in, you have a quick risk of fire.

If you want to replace the bong water with anything, look around online for these "energy drinks" that are advertised as bong water. It's said they help add flavor while having a only a few properties in them, thus not running the risk of losing your marijuana potency when the smoke is mixed with it. Don't know much about them myself, only heard alot from a friend, he bought some.