Also a non-drinker here; La Casera or mosto are the two best. You can get a mosto anywhere, and I actually like to mix the two together. Remember that if you're in an out-of-the-way bar, they may have the local version of La Casera...just ask for a "gaseosa" and you're set.
Of course for the breakfast crowd, instead of café you'll be taking in your daily dose of Cola Cao...¡ñam ñam!
By the way, WB: You CAN get Fanta in the US now...check out the thread that Wolf started in the non-Spain forum heralding this blessed event!!