Mosteiro dos Jerónimos
Praça do Império, Belém
Secular since the 1800's, Jerónimos Monastery is designated a UNESCO World Heritage Site because of the incredible Manueline-style architecture, all done in limestone over the course of more than one hundred years. It's a required tourist destination because it's so photogenic (sculptures and intricate design elements are carved into every detail), but the grounds are enormous, so particularly if you come by on a weekday, you'll have plenty of moments to yourself. Photos: Paulo Valdivieso, Shadowgate
Torre de Belém
Av. Brasília, Belém
This 16th-century tower, one of the city's most iconic landmarks, sits along the Tagus river. While the four-story structure was originally built to defend the capital against invaders, it has also welcomed Portuguese sailors home since the age of the great explorers; now, you can wander through its sparse inner chambers (littles, especially, will have fun exploring) and drink in the stunning Moorish architecture.