Plan a visit to Castillo San Cristóbal. One of the fortresses built by the Spanish to defend Puerto Rico. Explore the city walls and enjoy the amazing views.
The other Spanish fortress in San Juan is Castillo San Felipe del Morro, or El Morro. Where you can also visit the Cuartel de Ballajá which is now the Museo de las Américas.
One amazing thing that you can't miss in San Juan is a food tour! You learn so much about the area's history and culture through its food and tasting!
Visit the Museo de Arte de la Puerto Rico in Santurce, which is a hip and trendy area in San Juan. This art museum celebrates and showcases Puerto Rican and Caribbean art.
Santurce is home to Ocean Park beach (Playa Último Trolley), a public beach that's laid-back and not very touristy. This is the perfect spot for a family-friendly dip.
This day trip includes snorkeling stops, lunch, drinks, music on the boat, and a stop at Flamenco Beach on Culebra, one of the best beaches in the world.
I recommend strolling down the tree-lined Paseo de la Princesa in the evening and then walking along the Paseo del Morro to watch the sun set over the Bay of San Juan.