Visitors who prefer a tour with commentary can book a guide, but I would recommend wandering around the fortress and stopping occasionally to read information posted throughout and take in dusty exhibits on the history of the fort and of San Juan.
A small gift shop offers cookbooks, leather goods stamped with images of the fort, and coquis (Puerto Rico's famous tree frogs), as well as postcards, T-shirts, and bottles of water.
If you also plan on visiting El Morro, save a buck and buy a ticket to both fortresses (valid for 7 days including the day of purchase).
On a side note, wear sunscreen or cover up. I don't normally burn, but I came away from the few hours I spent at the fortress with the back of my hands quite red. Apparently, Puerto Rico is too close to the Equator to do without UVA protection.
While El Morro protected San Juan from attacks by sea, Fort San Cristóbal was built to protect the city from land attacks. Its construction started in 1634, after the Dutch successfully—though briefly—took the city by land. By the late 1700s, San Cristóbal had become massive, with walls that encompassed the old city. As part of the San Juan National Historic Site, Fort San Cristóbal is a UNESCO World Heritage Site and a U.S. National Historic Site. Visitors can explore the tunnels and dungeon, walk the walls with their garitas (sentry boxes) and see the Santa Barbara chapel.