I'm not really sold on the safety of stevia. What little research has been done on the herb found reproductive tract/sexual organ birth defects in rodents. While there has been a history of the herb's safe usage in folk medicine, the common usage was never as a daily or concentrated sweetener.
"Natural" doesn't really mean anything, because hemlock is natural, and artificial ingredients require more strict safety controls than natural ones. Artificial sweeteners have to be "proven safe" before they can be marketed, but "natural" products have to be proven unsafe before they can be taken off the market.
For that reason, I'm more comfortable using Splenda than Stevia (I personally don't have problems with aspartame either, but I know there's a lot more anectdotal problems with aspartame - but perhaps only because Splenda is newer).
You could try xylitol. It's a fruit sugar, so it's as natural as stevia (which is still very heavily processed, so I use the world "natural" a bit loosely)
Xylitol is a fruit sugar (also called a sugar alcohol or fruit alcohol) so it can have a laxative effect (as can any of the -ol sweeteners, or for that matter so can a large amount of fruit, but that's as much do to the fiber and the water as the fruit sugars).
In addition to the powdered drink mixes, there is MiO. It's marketed as a "water enhancer" and it has a better reputation than the powders, which I also think is silly (because it contains artificial sweeteners and is heavily colored). However, it's very easy to use a lot less. With powders, using less than the full packet is a bit inconvenient.
I also use natural and artificial extracts and flavoring oils. Extracts are less concentrated, and are usually in a water base (like the vanilla, mint, and almond extracts you buy for baking. You can find many other flavors like rootbeer, cherry, orange, lemon, raspberry - some are natural and some are artificial and some are imitation (imitation doesn't necessarily mean artificial). Vanillin is a natural extract but it's used as a vanilla substitute so it's called imitation vanilla flavoring, even though the source is natural. Sort of like carob being imitation chocolate.
I also use LorAnn Gourmet oils (tiny little square bottles used primarily for candymaking). Most are natural (the bottle will say) and are so concentrated that dipping a toothpick in the bottle and then stirring your drink with the toothpick is enough to give a nice amount of flavor (if it's not enough, I stick the clean end into the oil, then dip and stir again). You still have to add sweetener if you want it, but for the citrus flavors I usually don't. Or when I use it to flavor lemon lime diet soda.
Every time I go to the bulk store I buy a bottle ($1.50). A lot of grocery stores and craft stores sell the candymaking oils too. The flavor assortment is amazing (so far I've collected mandarin orange, key lime, cheesecake, raspberry, cherry, pistachio, caramel, and pecan).
The cheesecake smells foul, but it really tastes great in yogurt (using the toothpick dip and stir).