Heat the sugar with 1/4 cup cold water until it comes to a boil and sugar dissolves. Pour in remaining cold water; let cool before using. This is your simple syrup.
Mix cooled simple syrup with lime juice, pineapple juice, Campari and rum in a chilled bowl, or chill in the fridge until ready to serve.
To serve: Add sparkling wine, a large ice cube and citrus garnishes. Serve with a ladle into small cups.