To change up the flavor in an ordinary cake mix, substitute an equal of amount of any carbonated SODA that you want for the other liquid called for on the box!  ... Then, if you have leftover soda, you can mix that with some canned vanilla or chocolate frosting to use as a drizzle glaze on top.

Suggestions:

chocolate cake:  Dr. Pepper, coke, orange soda, root beer
​yellow cake: lemon-lime soda, orange soda
​white cake: root beer, lemon-lime, orange, grape