Is there anything I can add to/change about a store bought white or yellow cake mix to take it from ordinary to extraordinary?
There are a couple of cookbooks by Anne Byrn who has built a franchise on this subject. Her best known book (and the one I have) is titled The Cake Mix Doctor. written specifically on this subject.
Go to her website http://www.cakemixdoctor.com as there are several recipes and articles as well a listing of her books.
Using different spices, maybe a pumpkin pie spice added to vanilla cake, will give your cake a completely new and interesting flavor. Using citrus zests will spice things up as well! I have also used different flavored liquid extracts, added these to icing can really add that extra something special to the completed cake.
All answers provide great ideas but I'll take Jginger's spice recommendation a step further. I often use what I call "subtle exotic" spice combinations which is simply a fancy way of saying unusual or ethnic-influenced combinations. For example, Black pepper with dark chocolate; chili pepper and cinnamon; or cardamom. These types of spice combinations aren't typical in a cake and will provide a deliciously memorable flavor for your guests. A recent unusual combination I just tasted was a spiced gin cupcake. The baker used a basic cake recipe then infused spices and gin into the batter and frosting. A boxed cake could be that basic recipe.