Even if you managed to kick the regular soda habit and are no longer destroying your health with high fructose corn syrup, is it really wise to drink diet sodas? There is controversy about that topic with some saying both Aspartame and Splenda have potential hidden dangers. Until the science shakes itself out, it might be a great idea to drop soda all together.
As a long time soda junkie, I know that isn't easy. One thing that can work well if plain water bores you to tears is herbal tea. A type of tea I highly recommend is Cinnamon Spice tea. Two excellent brands are Market Spice Cinnamon Orange Tea or Country Spice Cinnamon Orange Tea. They are naturally sweet and require zero additional sweetener. How do they do it? Apparently it is the orange and cinnamon oils they add to the tea mix.
I simply brew up a big batch and put it in large swing top bottles, grab one when heading out to work and sip it throughout the day. This tea is so strong you can usually get three batches out of the same tea leaves. Using a French Press, I add 1/4 - 1/3 cup of loose tea and add hot water. Steep about 10 minutes then add more water. Each "pressing" will be a little weaker, but keep adding to a large pitcher. After you have exhausted the tea mix, add cold water to taste. That's it!
Keep in mind not all cinnamon orange teas will be naturally sweet like Market Spice or Country Spice, but I have found some at small tea shops that have a similar flavor and sweetness level. In Colorado Springs, Laura Lee's Teas carries it in regular and decaf. (I use decaf.) Ask the proprietor of your local tea shop if they have a tea that is a naturally sweet cinnamon orange tea and give it a try. If you find yourself reaching for sweetener, hop online and order yourself some of the Market Spice or Country Spice tea.