An Italian Soda is made by adding 1 shot of flavor syrup to 12 ounces of club soda.
Tonic water will leave you with a drink reminiscent of turpentine.

A variation of this drink is a cremosa, which is basically an Italian soda with cream added, topped with vanilla whipped cream. It's taste resembles that of an ice cream float.

In my experience, the best Italian Soda flavors are kiwi/lime, strawberry/kiwi, blackberry/cherry, and coffee (seriously... it tastes like kahlua and soda)
LaggedyAnne says she likes amaretto... FREAK! ;)
Never tried it, It'd be interesting though. I think I shall next time I want a cold drink