My wife is a snob when it comes to buying soda on the road. She'll crave a Diet Coke and insist we go through the McDonald's drive thru rather than get a fountain soda at the gas station. Apparently the flavor is stronger. Whatever.
Find a way to sneak a Pepsi or RC or whatever in something she would not know and tell her it is her precious Diet Coke. A few hours after she happily finishes it, come clean about it and she won't be able to be so high maintenance about it again.
My office did a blind taste challenge of Coke/Pepsi/RC. Most of us failed miserably in our predictions, including a couple of those coke snobs. It was awesome.