All drinks without an energy value, i.e. that contain no calories or very few calories, can be used during the day in an amount up to 600ml. This includes any of the following: 

Tea or coffee with no added milk or sugar

Diet soft drinks such as Coke Zero, Diet Coke, Pepsi Max, etc.

Herbal tea

Sparkling water

Sugar-free energy drinks such as sugar-free Red Bull

There are so many other ways to “spice” up your water so if you are looking for a change make sure you try the sugar free water drops (Vital Zing here in NZ), adding lemon or ginger to both hot and cold water, or even using a vitamin tablet like a Berocca just to change things up!