This huge fort was built in 1533 by the Spanish as a way to protect San Juan. It took over 150 years to build, and today it is both a world heritage site and a U.S. National Park. Kids will enjoy exploring the fort. Try to get them to imagine the huge squares filled with soldiers, cannons peering out to the sea, and what it would be like to wear a full uniform in the intense Puerto Rican heat. There's lots of room to run here, and some incredible views.
Be forewarned that this is not a stroller friendly attraction - there are lots of stairs and most areas of the fort are in full sun.