You can also try the Diet Cake. You dont follow the recipe on the boxed cake mix, you just pour the cake mix and one can of diet soda in a bowl and mix together then bake for the time on the box. You may have to keep an eye on it though.
Chocolate cake mix and diet dr pepper makes for an awesome combo, and in my experience sprite (or lemon lime) and vanilla cake mixes dont go well
Oh, and you can use fat free cool whip as icing too. Its actually what I prefer, sometimes I mix a box of jello pudding into it and use that for a sweeter flavor on the icing bit.
^-^