Tonic water

Simple as?:

cleaning and sanitizing keg
adding water
putting on CO2 around 20psi for 35F

Would like to have some on tap as the wife loves flavored sparkling water.

Are you referring to Seltzer Water, Club Soda, or Tonic Water? Tonic water has quinine in it, so if you're not adding quinine, it's not tonic water. Seltzer water and club soda are more the same thing, although club soda specifically denotes carbonated water with higher mineral content (I personally can't usually tell the difference, I'd be inclined to say that all club soda is seltzer water but not all selzter water is club soda if that makes any sense).

Hmm, I guess what I'm going for is Seltzer then, honestly didn't know there was a difference.

If you like, pick up all three (or even all four, and include "mineral water") from your local store and try them side by side. I can easily tell the difference between "mineral water", and the other two plain carbonated waters easily, as mineral water a) isn't always carbonated and b) often has a SUBSTANTIAL mineral content (there's a German brand, Gerolsteiner, that I enjoy that's ~1800ppm alkalinity or something like that). Club soda and seltzer are a little tougher. But the quinine makes tonic water taste completely different than the other three. Quinine is very bitter, and to balance tonic water often has a lot of sugar added as well. If you've tried making a gin and tonic with seltzer instead of tonic water, you'll know why it doesn't work.

Fill keg with clean filtered water, carb to 4.0 levels CO2. Enjoy

We have carb'd water on tap all the time. Half water half juice makes a great drink. The soda syrups from soda stream are awesome too

seltzer, club soda, sparkling mineral water, and tonic are all different.

what you made is seltzer. pure(ish) water, carbonated.

if you take seltzer and add sodium bicarbonate and some other things, you'll get club soda.

mineral water is water from a natural spring or well. sometimes carbonation is natural and sometimes not.

tonic is widely different than the rest. tonic is more like a soda in that it has a good amount of sugar and some flavorings, one of them being quinine.

And if you want *tonic*, this is what I used: http://www.jeffreymorgenthaler.com/2008/how-to-make-your-own-tonic-water/

I've made changes to it (like I'm working on changing the agave to stevia), but everyone who's tasted it likes it.