Pour water into slow cooker. Heat on High 1–2 hours. (Or heat water in tea kettle and pour into slow cooker.) Turn cooker to Low to keep hot for hours.
Stir in hot chocolate mix until blended.
Arrange stir-ins in small bowls.
Instruct guests to place approximately 1 Tbsp. of desired stir-in in mug before ladling hot chocolate in. Stir well.<