Are you reading The BFG by Roald Dahl?
Print this label and attach it to a 2 liter bottle of crème soda with a drop or two of green food coloring. Voila! Frobscottle!!
The kids will go crazy and then will write you pages and pages of descriptive writing based on what they see, hear, feel, taste, and smell.
Whatever you do, don't tell them that its plain 'ole crème soda.