This stunning thirst-quencher gets its deep red color from dried hibiscus flowers, which are sold as flor de jamaica at Latin grocers. The hibiscus mixture can be made up to 3 days in advance; add seltzer just before serving.
Bring ginger, dried hibiscus, honey, cinnamon stick, and 2 cups water to a boil. Reduce heat to medium-low; simmer 5 minutes. Remove from heat and let steep 10 minutes. Strain through a sieve into a heatproof container. Add 2 cups water; refrigerate until cold. Pour halfway up ice-filled glasses; top with seltzer.
To spike it: Add 1 ounce light rum per serving, before the seltzer.