If you have the time, squeezing your own grapefruit juice really amps up the flavor, but don't sweat it if you have to opt for the supermarket stuff. Another nice touch: Plan ahead of time and fill a plastic bowl with water and cranberries, then pop it in the freezer. You'll be left with a jumbo, sparkly ice cube that will melt slowly without diluting the punch—and look super pretty, too.