Adding soda may sound strange, but it sure does help. Try replacing it with dark soda like Coca Cola and Thums Up. It amplifies the flavor tremendously.
Lemon blends well with cake mix. The tartness will help cut the inherent sweetness of the mix while giving it a new flavor. However, do not add lemon to chocolate cakes.
Coffee and chocolate blends perfectly. Add coffee to your chocolate cake mix for the yummiest taste ever. Coffee is another suitable replacement for water.
Add fruit curd between the layers of the cake. It adds a tart flavor for a break from that sugary frosting, as well as another texture.
Since boxed cake mixes tend to be lighter in consistency, add one extra egg for that richer, thicker and moist cake taste.
Butter has a lot more flavor than vegetable oil, and it makes for a much better texture. Replace the oil with an equal amount of either softened or melted butter.