What is it? Rum! The island is full of distilleries putting molasses and sugarcane juice to good use. Notables include giant Bacardi, who offer free tours of their "Cathedral of Rum" (so christened by the island's governor back in the 1950s); Don Q, probably the most popular local product; and Edmundo B. Fernández, Inc., the island's oldest operation, founded by the excellently named Fernando Fernández.
The details: This is a great opportunity to discover what rum you actually like. On the light end, you have ron blanco (white rum), ideal for mixing with Coca Cola, coconut water, or whatever else you can hustle up. Ron añejo, meanwhile, puts the spirit to rest in wooden barrels, à la bourbon, to impart vanilla and caramel notes; this is the thinking-man's rum, best savored neat. E.B. Fernández's Ron de Barrilito is the aged label to search out, unless the island happens to achieve independence during your vacation, in which case you'll want to sample their Freedom Barrel: a single cask whose opening's been awaited since 1942.