What can I add to white or chocolate cake mix to jazz up the flavor? Other than alcohol.

Everything you ever wanted to know to bake,assemble and decorate tiered/stacked or layer cakes:

http://www.cakecentral.com/cake-decorating-ftopict-605188-.html

Above thread has popular CC recipes for doctored cake mix, American buttercream and fondant (plus so much more).

The WASC cake recipe can be made with any flavor of cake mix and complementary flavorings. Can also use: white chocolate, fruit or lemon juice, frozen berries, coffee and more.

HTH

Besides my *original* WASC cake recipe that is highly used on this site there are many flavor/extract combinations one can use. Don't be afraid to try some new combinations.
You can add different yogurt flavors; use fruit juice or soda pop instead of water; the flavored coffee creamers are also a good way to change things around.

I always add 4 eggs, 1/2 cup oil, 1 cup water and a box of instant pudding to any flavor box mix.