I don't drink soda, so that is an easy challenge to accept. The only exceptions are if I'm at a social event and I have to drive, or I have a stomach bug and ginger ale works wonders.
What would be a crazier challenge is not buying any processed cookies, pastries, candy, etc and see how that affects certain companies. Sugar is a multi billion dollar industry. They come down hard on anyone who dares try to enlighten the public about how dangerous consuming too much sugar is. Even Michael Pollan noted their bullying tactics in one of his books. Sugar addiction is just as real as any other addiction. So I would challenge everyone to make their own cookies and pastries,etc for 30 days. Yes, sugar is still involved, but you can substitute honey and maple syrup, coconut sugar, beet sugar and other natural sugars for cane sugar. And you are not buying processed food with a zillion additives in it.