Old San Juan is the city's main attraction — a 7-block walled town with the two great Spanish forts at either end, half a dozen churches, a clutch of small museums, and streets paved in blue cobblestones cast from the slag of European iron foundries. You can walk it in a long day; you should spend two if you want to do the forts properly.
Beyond the old city, the Museo de Arte de Puerto Rico in Santurce holds the strongest art collection on the island, and the rum distillery in Cataño is a half-day excursion by ferry. The neighbourhoods of Condado, Ocean Park, and Isla Verde are where the beach scene happens. La Perla, just outside the north city walls, is a tight-knit working-class barrio that became famous for "Despacito" — visit during the day if you want to see it.