Half fill a hi ball glass with sprite. Add vodka and creme de menth (mixed with green food colour/or use green creme). Top up with sprite and garnish with fresh mint.
If it works well the creme de menth should form a green layer (lagoon) in the bottom of the glass. or the cocktail can be stirred to form an entirely green cocktail.
Other users who liked a Green lagoon (my way) also liked: