We chose a diet root beer soda to avoid excess sugar and limit the calories. The amount of diet root beer is not limited because it has "zero" calories.
Beverage. Make sure you carefully combine the soda and ice cream. The reaction causes bubbles to form and a rapid rise in foam, which is part of the fun and can get messy. Use 1/4 cup light vanilla ice cream. Add a straw and a spoon and enjoy.