What are the most worthwhile things to do in San Juan on a first visit?

Most visitors treat the forts as one stop. The National Park Service treats them as one ticket, which is not the same thing. The $10 per-person entrance pass admits you to both fortifications and remains valid for 24 hours from purchase, and the walk between them crosses the length of the old city. El Morro guards the harbor mouth on the headland; San Cristóbal faces inland and to the east, built against attack by land. Doing both in a single afternoon means rushing the second one.

El Morro's construction began in 1539, and what stands today is the accumulated result of nearly two and a half centuries of Spanish engineering layered on that start. It is why the fort reads as a stack rather than a building: ramps, tunnels, batteries and a lighthouse at the top, with the Atlantic on three sides.

Beyond the ticketed ground, the old city itself is the attraction, and the World Heritage title names what gives it weight. UNESCO inscribed the property in 1983, by decision 07COM VIII at the seventh session of the World Heritage Committee, under cultural criterion (vi). The property is titled La Fortaleza and San Juan National Historic Site in Puerto Rico — La Fortaleza being the mansion that still serves as the official residence of Puerto Rico's governor, which means the first-named element of a World Heritage site here is a working seat of government rather than a museum.

The city wall runs from the harbor side of the old town around the base of El Morro, and the walkway beneath it sits close enough to the water that the stone is salt-stained at shoulder height. Inside the forts the temperature drops noticeably in the vaulted casemates — thick masonry and cross-ventilation doing what no air conditioning is installed to do.

Evening is possible but not standard. The Superintendent's Compendium restricts public use of both castillos to 9:00 a.m. to 5:00 p.m., and allows later access only during special evening programs — so an after-dark visit depends on a scheduled program existing, not on simply showing up. The holiday closures are absolute: the site shuts on Thanksgiving, Christmas and New Year's Day, three of the days when cruise arrivals are heaviest.

• Day one, from nine — Start at San Cristóbal, the inland fort, and buy the pass there. It is the quieter of the two at opening.
• Day one, afternoon — Walk the old town between the two forts. The pass is still running, so nothing is wasted by stopping.
• Day two, from nine — Use the second half of the 24-hour window at El Morro, on the headland, before the midday heat reaches the upper batteries.

Visitors over 16 must hold the $10 pass; children 15 and under enter free. Both forts are multi-level stone structures with ramps and stairs and very little shade above the lower batteries. If your visit falls between June 1 and November 30, the Atlantic hurricane season as defined by the National Hurricane Center, a passing system can close outdoor areas at short notice even when the city itself is unaffected. La Fortaleza, despite leading the World Heritage title, is a functioning official residence and is not entered on the Park Service pass.

The when-to-visit answer in this group sets out the seasonal and climate picture behind these dates; the food answer explains how to eat around a nine-to-five fort day.