Climb the ramparts and stroll the grounds at San Cristobal Castle, the largest fortification built by the Spanish in the New World. Built to protect against land-based attacks on San Juan, it covered 27 acres when it was finished in 1783.
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.