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!