This is a staple of many tiki cocktail recipes, and there are probably as many variations on how to make it as there are on most tiki cocktails. But here's a version we find to be authentic-tasting and not impossible to make.
3 cinnamon sticks
1 cup unrefined sugar
1 cup water
2 cups white grapefruit juice (fresh-squeezed or canned)
Smash cinnamon sticks in a zip lock bag until coarsely broken. Add cinnamon stick pieces to sugar and water in a saucepan and bring to a boil. Let simmer for 10 minutes. Remove from heat and let steep for 15 minutes. Strain cinnamon syrup into a mason jar with a mesh strainer. Let cool. (This step is called Don's Spices #4, for the serious tiki-heads out there.)
Add grapefruit juice to cinnamon mixture and shake. Mixture can be sealed and kept in the refrigerator for up to 5 days,