Try using melted butter (cooled) instead of oil. This will increase the fat content and flavor which will result in a slightly more dense cake crumb!
Add Butter instead of Oil
Use Mix-Ins: Chocolate Chips Sprinkles Fruit Jello
Add Coffee to chocolate cake mix for richer flavor
use an Extra Egg or Egg White to increase moisture
for even more ways to add flavor to a box cake, swipe up!