The Spumoni Cocktail

A low-proof G&T-style drink calling on Campari instead of Gin. It's really delicious!
1 oz Campari
1.5 - 2 oz fresh Grapefruit Juice
3-4 oz cold tonic water
Add ice to a high ball glass, topping it with the Campari and grapefruit juice. Stir briefly and then top with tonic water, stirring gently. Garnish with a slice of grapefruit.