Very refreshing. This is also really nice made with freshly squeezed orange juice, or you can use lemon juice and substitute the Campari for Cointreau.
500ml pink grapefruit juice
150g white sugar
Combine the juice and sugar together in a pot and heat gently until the sugar has dissolved. Do not boil. Remove from heat, add the Campari and pour into a wide, flat container that will fit in the freezer. Place in freezer and about every hour bring it out, whisk up all the shards and refreeze until set (it will probably take a few hours, but freezers vary). Chill glasses before serving.